YEARS_AGOfunctions to calculate the number of years it has been since a specific date. The date can be either given as an information or a text in ISO8601 format.
If your organisation only offers services to people over the age of 18, you can attach the following condition to a process rule:
p is Person
YEARS_AGO(p.date_of_birth) < 18
If today is 11 November, 2020, and the person who is applying for a service was born on 15 November, 2002, the process rule will raise an issue.
If that same person returns on 15 November, 2020, the condition evaluates to True, so the case can continue without issues.