There have been many post about computing the age. Have you searched for any of these posts?co
The coding needed for this task is easier if one has a field to hold the result of calculating age, one field with one script. But since the check box field has no way to automatically compute, only when there is a change to the field, one needs to add a script to the "SYSACTUALDATE" and "BIRTHDATE" fields with only a minor variation. If a calculation field for the age is used, then there is only one script at one place that is used. The calculated age field serves as validation of calculation as well. You will need to update the code each year so having only one script to update may eliminate the error of not updating both scripts.
Validate Age is a sample form with your approach;