public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
    public double Age { get; set; }
    public double FatherAge { get; set; }
}

public class HomeController : Controller
{
    public ActionResult Index( )
    {
        ViewBag.Message = "Welcome to ASP.NET MVC!";

        return View();
    }

    public ActionResult About( )
    {
        return View();
    }

    public ActionResult Create( )
    {
        return View( new User() );
    }
}

public class User
{
    public int Id { get; set; }

    [Required]
    [StringLength(30)]
    public string Name { get; set; }

    [Range(1,100)]
    public double Age { get; set; }

    [Range( 1 , 100 )]
    public double FatherAge { get; set; }
}

namespace AgeValidator
{
    public class AgeCompareAttribute : ValidationAttribute , System.Web.Mvc.IClientValidatable
    {
        #region IClientValidatable Members

        public IEnumerable<ModelClientValidationRule> GetClientValidationRules( ModelMetadata metadata , ControllerContext context )
        {
            throw new NotImplementedException();
        }

        #endregion
    }
}

namespace AgeValidator
{
    public class AgeCompareAttribute : ValidationAttribute , System.Web.Mvc.IClientValidatable
    {
        private const string DefaultErrorMessageFormatString = "'{0}' must be lower than {1} by 15 year a least.";

        private string _DependentPropertyDisplayName = null;
        public string DependentProperty { get; set; }


        /// <summary>
        /// Construcror
        /// </summary>
        /// <param name="DependentProperty">Model Dependent Property</param>
        public AgeCompareAttribute( string DependentProperty )
            : base( DefaultErrorMessageFormatString )
        {
            this.DependentProperty = DependentProperty;
        }

        /// <summary>
        /// Format Error Message 
        /// </summary>
        /// <param name="name"></param>
        /// <returns></returns>
        public override string FormatErrorMessage( string name )
        {
            return String.Format( DefaultErrorMessageFormatString , name  , ( this._DependentPropertyDisplayName ?? this.DependentProperty ) );
        }

        protected override ValidationResult IsValid( object value , ValidationContext validationContext )
        {
            return ValidationResult.Success;
        }

        #region IClientValidatable Members

        public IEnumerable<ModelClientValidationRule> GetClientValidationRules( ModelMetadata metadata , ControllerContext context )
        {
            throw new NotImplementedException();
        }

        #endregion
    }
}

protected override ValidationResult IsValid( object value , ValidationContext validationContext )
       {
           // Get Depented Proprety
           var containerType = validationContext.ObjectInstance.GetType();
           var field = containerType.GetProperty( this.DependentProperty );

           if ( field != null )
           {
               this._DependentPropertyDisplayName = field.Name;
               var dependentvalue = field.GetValue( validationContext.ObjectInstance , null );

               if ( dependentvalue != null && value != null )
               {
                   double _value = Convert.ToDouble(value);
                   double _dependentvalue = Convert.ToDouble(dependentvalue);
                   
                   //validate
                   if((_dependentvalue - _value) < 15)
                       return new ValidationResult( FormatErrorMessage( validationContext.DisplayName ) , new[ ] { validationContext.MemberName } );
               }
           }
           return ValidationResult.Success;
       }