This calculation is needlessly complex if you just need to know the person's current age in years.
Let ( [ B = DOB ;
T = Get ( CurrentDate )
Year ( T ) - Year ( B ) - ( T < Date ( Month ( B ) ; Day ( B ) ; Year ( T ) ) )
As long as this is an unstored calculation and DOB is a field of type date, not text, this will give you the person's age in years. This should be an unstored calculation field in order to make sure that the value returned will update automatically with the passage of time.
With this as a calculation field that specifies a Number result type, you can enter find mode and specify <21 in this calculation field to perform a find for all records where the age is less than 21.
Awesome. Now how would I create this ? Where do I put in the calculation you've given me?
You would go to Manage | Database | fields and create a new field of type calculation and enter this calculation as the expression for that field. You'd then select "Number" from the result type drop down and click the storage options button to select "do not store..." as the storage option for it.