This is the Xamarin Forms date picker extension. It can set border width, color and radius, also it can add PlaceHolder and MinimumAge

RoundBorderDatePicker.cs

using System;
using Xamarin.Forms;

namespace MyApp
{
    public interface ICanBeValidated
    {
        Action<bool> ValidateChange { get; set; }

        bool IsValidated { get; set; }

        Xamarin.Forms.Color BorderColor { get; set; }

        double BorderWidth { get; set; }

        double BorderRadius { get; set; }
    }
    
    public class RoundBorderDatePicker : DatePicker, ICanBeValidated
    {
        public RoundBorderDatePicker ()
        {
            // It seems that all the picker need to validated
            IsValidated = false;
            BorderColor = Color.Red;
            this.DateSelected += RoundBorderDatePicker_DateSelected;
        }

        public static readonly BindableProperty BorderWidthProperty = BindableProperty.Create (
                                                                          "BorderWidth",
                                                                          typeof(double),
                                                                          typeof(RoundBorderDatePicker),
                                                                          2.0);

        public double BorderWidth { 
            get {
                return (double)GetValue (BorderWidthProperty);
            }
            set {
                SetValue (BorderWidthProperty, value);
            }
        }

        public static readonly BindableProperty BorderColorProperty = BindableProperty.Create (
                                                                          "BorderColor",
                                                                          typeof(Color),
                                                                          typeof(RoundBorderDatePicker),
                                                                          Color.FromHex ("#807A79"));

        public Color BorderColor { 
            get {
                return (Color)GetValue (BorderColorProperty);
            }
            set {
                SetValue (BorderColorProperty, value);
            }
        }

        public static readonly BindableProperty BorderRadiusProperty = BindableProperty.Create (
                                                                          "BorderRadius",
                                                                          typeof(double),
                                                                          typeof(RoundBorderDatePicker),
                                                                          20.0);

        public double BorderRadius { 
            get {
                return (double)GetValue (BorderRadiusProperty);
            }
            set {
                SetValue (BorderRadiusProperty, value);
            }
        }

        public static readonly BindableProperty PlaceHolderProperty = BindableProperty.Create (
            "PlaceHolder",
            typeof(string),
            typeof(RoundBorderDatePicker),
            "");

        public string PlaceHolder { 
            get {
                return (string)GetValue (PlaceHolderProperty);
            }
            set {
                SetValue (PlaceHolderProperty, value);
            }
        }

        public static readonly BindableProperty MinimumAgeProperty = BindableProperty.Create (
            "MinimumAge",
            typeof(int),
            typeof(RoundBorderDatePicker),
            18);

        public int MinimumAge { 
            get {
                return (int)GetValue (MinimumAgeProperty);
            }
            set {
                SetValue (MinimumAgeProperty, value);
            }
        }

        public Action<bool> ValidateChange { get; set; }

        public bool IsValidated { get; set; }

        void RoundBorderDatePicker_DateSelected (object sender, DateChangedEventArgs e)
        {
            if (e.NewDate.Date != DateTime.Now.Date && e.NewDate.Year < (DateTime.Now.Year - MinimumAge)) {
                IsValidated = true;
                if (ValidateChange != null)
                    ValidateChange (true);
                BorderColor = Color.FromHex ("#807A79");
            } else {
                IsValidated = false;
                if (ValidateChange != null)
                    ValidateChange (false);
                BorderColor = Color.Red;
            }
        }
    }
}

RoundBorderDatePickerRendererDroid.cs

using System;
using Xamarin.Forms;
using Xamarin.Forms.Platform.Android;

[assembly: ExportRenderer (typeof(RoundBorderDatePicker), typeof(MyApp.Droid.RoundBorderDatePickerRendererDroid))]
namespace MyApp.Droid
{
    public class RoundBorderDatePickerRendererDroid : DatePickerRenderer
    {
        RoundBorderHelper _helper;

        protected override void OnElementChanged (ElementChangedEventArgs<DatePicker> e)
        {
            base.OnElementChanged (e);
            if (Control != null && Element is ICanBeValidated)
                _helper = new RoundBorderHelper (Control, Element as ICanBeValidated);
                Control.Text = (Element as RoundBorderDatePicker).PlaceHolder;     
                (Element as ICanBeValidated).ValidateChange = (obj) => {
                    if(!obj)
                        Control.Text = (Element as RoundBorderDatePicker).PlaceHolder;                    
                };
        }

        protected override void OnElementPropertyChanged (object sender, System.ComponentModel.PropertyChangedEventArgs e)
        {
            base.OnElementPropertyChanged (sender, e);
            _helper.UpdateBorderByPropertyName (e.PropertyName);
        }
    }
}

RoundBorderDatePickerRendereriOS.cs

using System;
using Xamarin.Forms;
using Xamarin.Forms.Platform.iOS;

[assembly: ExportRenderer (typeof(RoundBorderDatePicker), typeof(MyApp.iOS.RoundBorderDatePickerRendereriOS))]
namespace MyApp.iOS
{
    public class RoundBorderDatePickerRendereriOS : DatePickerRenderer
    {
        RoundBorderHelper _helper;

        protected override void OnElementChanged (ElementChangedEventArgs<DatePicker> e)
        {
            base.OnElementChanged (e);
            if (Control != null && Element is ICanBeValidated) {
                _helper = new RoundBorderHelper (Control, Element as ICanBeValidated);
                Control.Text = (Element as RoundBorderDatePicker).PlaceHolder;   
                (Element as ICanBeValidated).ValidateChange = (obj) => {
                    if(!obj)
                        Control.Text = (Element as RoundBorderDatePicker).PlaceHolder;                    
                };
            }
        }

        protected override void OnElementPropertyChanged (object sender, System.ComponentModel.PropertyChangedEventArgs e)
        {
            base.OnElementPropertyChanged (sender, e);
            _helper.UpdateBorderByPropertyName (e.PropertyName);
        }
    }
}

RoundBorderHelperDroid.cs

using System;
using Android.Views;
using Android.Graphics.Drawables;
using Xamarin.Forms.Platform.Android;

namespace MyAPP.Droid
{
    public class RoundBorderHelper
    {
        View _control;
        ICanBeValidated _element;

        public RoundBorderHelper (View control, ICanBeValidated element)
        {
            _control = control;
            _element = element;
            UpdateBorder ();
        }

        public void UpdateBorder ()
        {
            GradientDrawable gd = new GradientDrawable ();
            //gd.SetColor (_element.BackgroundColor.ToAndroid ());
            gd.SetStroke ((int)_element.BorderWidth * 2, _element.BorderColor.ToAndroid ());
            gd.SetCornerRadius ((float)_element.BorderRadius);
            _control.SetBackground (gd);
        }

        public void UpdateBorderByPropertyName (string propertyName)
        {
            switch (propertyName) {
            case "BorderColor":
            case "BorderRadius":
            case "BorderWidth":
                UpdateBorder ();
                break;
            default:
                return;
            }
        }
    }
}

RoundBorderHelperiOS.cs

using System;
using UIKit;
using Xamarin.Forms.Platform.iOS;

namespace MyApp.iOS
{
    public class RoundBorderHelper
    {
        UITextField _control;
        ICanBeValidated _element;

        public RoundBorderHelper (UITextField control, ICanBeValidated element)
        {
            _control = control;
            _element = element;

            _control.BorderStyle = UITextBorderStyle.RoundedRect;
            _control.ClipsToBounds = true;
            _control.Layer.MasksToBounds = true;
            UpdateBorder ();
        }

        public void UpdateBorder ()
        {
            _control.Layer.CornerRadius = (nfloat)_element.BorderRadius;
            _control.Layer.BorderColor = _element.BorderColor.ToCGColor ();
            _control.Layer.BorderWidth = (nfloat)_element.BorderWidth;

        }

        public void UpdateBorderByPropertyName (string propertyName)
        {
            switch (propertyName) {
            case "BorderColor":
            case "BorderRadius":
            case "BorderWidth":
                UpdateBorder ();
                break;
            default:
                return;
            }
        }
    }
}