YEARS_AGO functions to calculate the number of years it has been since a specific date.
If your organization only offers services to people over the age of 18, you can attach the following condition to a process rule:
p is PersonYEARS_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.