.NET Core Web API - IValidatableObject vs Custom validation attribute

Employee.cs

// Employee.cs - Model for holding employee information
public class Employee : IValidatableObject
{
    [Required]
    [StringLength(maximumLength: 250, MinimumLength = 10)]
    public string Name { get; set; }

    [Required]
    public DateTime DateOfBirth { get; set; }

    [EmailAddress(ErrorMessage = "Email address is not valid.")]
    public string Email { get; set; }

    [Phone(ErrorMessage = "Phone is not valid.")]
    public string Phone { get; set; }

    [Range(minimum: 50, maximum: 400,
        ErrorMessage = "Hourly salary does not fall within allowed range.")]
    [MinLegalSalaryRequired(minJuniorSalary: 50, minSeniorSalary: 200)]
    public double HourlySalary { get; set; }

    public IEnumerable<ValidationResult> Validate(ValidationContext validationContext)
    {
        var minAllowedBirthDate = DateTime.Today.AddYears(-65);
        if (minAllowedBirthDate > DateOfBirth)
        {
            yield return new ValidationResult("Birthdate is not valid.");
        }
    }
}

// MinLegalSalaryRequiredAttribute.cs - custom validation attribute
public class MinLegalSalaryRequiredAttribute : ValidationAttribute
{
    public MinLegalSalaryRequiredAttribute(double minJuniorSalary, double minSeniorSalary)
    {
        MinJuniorSalary = minJuniorSalary;
        MinSeniorSalary = minSeniorSalary;
    }

    public double MinJuniorSalary { get; }
    public double MinSeniorSalary { get; }
    public string GetErrorMessage() => $"Minimum hourly salary is not valid.";

    protected override ValidationResult IsValid(object value, ValidationContext validationContext)
    {
        var employee = (Employee)validationContext.ObjectInstance;
        var dateBeforeThirtyYears = DateTime.Today.AddYears(-30);
        var isOlderThanThirdyYears = employee.DateOfBirth <= dateBeforeThirtyYears;
        var hourlySalary = (double)value;

        var isValidSalary = isOlderThanThirdyYears
            ? hourlySalary >= MinSeniorSalary
            : hourlySalary >= MinJuniorSalary;

        return isValidSalary
            ? ValidationResult.Success
            : new ValidationResult(GetErrorMessage());
    }
}