[Required]
[MinimumAge(18, ErrorMessage = "must be 18 years old or up")]
[Display(Name = "Birth date")]
public DateTime birthdate { get; set; }

public class MinimumAgeAttribute : ValidationAttribute
{
    int _minimumAge;

    public MinimumAgeAttribute(int minimumAge)
    {
        _minimumAge = minimumAge;
    }

    public override bool IsValid(object value)
    {
        DateTime date;
        if (DateTime.TryParse(value.ToString(), out date))
        {
            return date.AddYears(_minimumAge) < DateTime.Now;
        }

        return false;
    }
}

if (DateTime.TryParse(value.ToString(), out date))