How do you apply field validation to a date field?

How do you apply field validation to a date field?

We are using Sign for remote e-signature of liability waivers. The signer must enter a date of birth, and if they are a minor their legal guardian must sign for them. However we have had numerous problems with signers returning an invalid date of birth (example - Jan-25-2025, which is absurd).

I want to validate that the date field is at least 13 years old before allowing a signature. I cannot for the life of me figure out a way to do that. Any help?