Sure. Use this formula:
[=FD(ADM('{TICKET DATE}',-216),'dd/MM/yyyy')]
ADM function adds Months, so 18 years x 12 months = 216 months. In this case I used -216 (negative) to subtract months from current ticket date.
This is 1 solution that could be modified to suit your needs.
Ensure you add a Default mapping to the following Rule:
MessageToDisplay: Customer must be Born on or before this date: [=FD(ADM('{TICKET DATE}',-216),'yyyy-MMM-dd')]
