Formula Field

@eradwan 

The formula would need to look like this:

if(DateDiff( [DateSigned1] ,[Date of Birth])>=6574,1,0)

Using the Date Signed field for todays date (data label is “DateSigned1”) and a text field with date field validation (data label is “Date of Birth”). The conditional logic will show the “Over 18 years” field when the value is 1 and the “Under 18 years” field when the value is 0. The formula field color is set to White, so it is invisible. I forgot to make the two outcome fields “read only”, I would recommend to do that as well.

Outcome with same date 18 years ago:

Outcome with other date, under 18 years:

Important: Due to leap year, in the formula 6574 or 6575 days could actually mean 18 years.