Azure B2C Sign up age restriction using custom policies - azure

Question

I'm trying to configure a B2C tenant using policies instead of user flows. For that, I'm using the SocialAndLocalAccount template as start base.
In the sign up page, I added a custom claim to ask the user about his/her birth date. If the user provides a date which make him/her an under age (+18), I would like to display a verification failed message (like the one you can set by using predicates) and prevent the user to be able to sign up by the create button being disabled.
So far, this is what I've got:
-I created two new claims to store a boolean value that would tell me if the user is under age or not and a second one to store the current time.
<ClaimType Id="systemDateTime">
<DisplayName>Today's date</DisplayName>
<DataType>dateTime</DataType>
</ClaimType>
<ClaimType Id="isNotUnderAge">
<DisplayName>Indicates whether user being under age or not</DisplayName>
<DataType>boolean</DataType>
<AdminHelpText>User must be over 18</AdminHelpText>
</ClaimType>
I added to ClaimsTransformation to get the current date and compare it to the selected one (date comparison):
<!-- Check user under age -->
<ClaimsTransformation Id="GetSystemDateTime" TransformationMethod="GetCurrentDateTime">
<OutputClaims>
<OutputClaim ClaimTypeReferenceId="systemDateTime" TransformationClaimType="currentDateTime" />
</OutputClaims>
</ClaimsTransformation>
<ClaimsTransformation Id="CheckBirthDateIsNotUnderAge" TransformationMethod="DateTimeComparison">
<InputClaims>
<InputClaim ClaimTypeReferenceId="birthDate" TransformationClaimType="firstDateTime" />
<InputClaim ClaimTypeReferenceId="systemDateTime" TransformationClaimType="secondDateTime" />
</InputClaims>
<InputParameters>
<InputParameter Id="operator" DataType="string" Value="later than" />
<InputParameter Id="timeSpanInSeconds" DataType="int" Value="568025136" />
</InputParameters>
<OutputClaims>
<OutputClaim ClaimTypeReferenceId="isNotUnderAge" TransformationClaimType="result" />
</OutputClaims>
</ClaimsTransformation>
Until this point, I think I'might be able to tell if the user is under age or not. From here on, I'm not sure how to continue in order to achieve my goal. I've been reading documentation but didn't come to a solution and I'm unsure that this would be the best approach.
Have you ever came across a similar restriction? If so, which is the best way to achieve this and where can I find any tips that help me resolve my issue?
Thank you very much!
EDIT 1:
I've found this question which explains a similar problem if not the same and it make me ask myself, is it possible to achieve this by using ClaimsTransformations and Predicates or is the REST API the only way to get the expected result?

Answer 1

It looks like you have the right idea as long as you want to blanket ban all users under 18. One option is you can use a validation technical profile to call AssertBooleanClaimIsEqualToValue transformation, which will display a custom error message on the sign-in page if the user is under 18 and won't let them continue. See Microsoft Documentation: Boolean Claim Transformations for example.
If you really want the Continue button to grey out, you could display isNotUnderage as a readonly claim and hide it with CSS, and then use JS to grey out the button based on its value.
The third option is to add an OrchestrationStep before presenting the user with your signup options with a precondition to skip if user is over 18 that will display a new Self-Asserted Technical Profile you can set up to display an error message (just a paragraph claim) and remove the continue button from. The downside here is that it requires you collect the user's age before offering them sign-up options which can complicate things.