asp net mvc custom validation attribute (checks if age is in allowed range based on date of birth)

Custom validation attribute

using System;
using System.ComponentModel.DataAnnotations;

namespace SampleProject.Validators
{
    public class AgeRangeAttribute : ValidationAttribute
    {
        private readonly int _minAge;
        private readonly int _maxAge;

        public AgeRangeAttribute(int minAge, int maxAge)
        {
            _minAge = minAge;
            _maxAge = maxAge;
            ErrorMessage = "{0} must be between {1} and {2}";
        }

        protected override ValidationResult IsValid(object value, ValidationContext validationContext)
        {
            if (!(value is DateTime))
            {
                throw new ValidationException("AgeRange is only valid for DateTime properties.");
            }

            var birthdate = (DateTime)value;

            var today = DateTime.Today;
            var age = today.Year - birthdate.Year;
            if (birthdate > today.AddYears(-age)) { age--; }

            if (age < _minAge || age > _maxAge)
            {
                return new ValidationResult(this.FormatErrorMessage(validationContext.MemberName));
            }

            return ValidationResult.Success;
        }

        public override string FormatErrorMessage(string name)
        {
            return string.Format(this.ErrorMessageString, name, _minAge, _maxAge);
        }
    }
}