RegisterPageViewModel.cs

 public class RegisterPageViewModel: INotifyPropertyChanged
  {
        ...
        bool AreFieldsValid()
        {
            bool isFirstNameValid = FirstName.Validate();
            bool isLastNameValid = LastName.Validate();
            bool isBirthDayValid = BirthDay.Validate();
            bool isPhoneNumberValid = PhoneNumber.Validate();
            bool isEmailValid = Email.Validate();
            bool isPasswordValid = Password.Validate();
            bool isTermsAndConditionValid = TermsAndCondition.Validate();

            return isFirstNameValid && isLastNameValid && isBirthDayValid
                   && isPhoneNumberValid && isEmailValid && isPasswordValid && isTermsAndConditionValid;
        }
  }