can we write a custom function in Approval rules (ICP)

Summary:

In ICP, we need to set approvals based on child's age/DOB(captured as an input value in the element linked to the Option).

Can we write a custom function ? or is there a way to fetch the requestor's dependent's Date of Birth while applying for the request.

Content (please ensure you mask any confidential information):

If child's age <= 18 years, no approvals.

If Child's age 18 years, go for approvals.

Version (include the version you are using, if applicable):

23D

Code Snippet (add any code snippets that support your topic, if applicable):