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JotForm Support Manager
This can be possible using calculations along with conditions.
Example: http://www.jotformpro.com/form/50644512494960 (Submit button appears when age is greater than 18. you can also show any text so the user will be aware of it.)
Clone that form so you can figure out the setup.
The form was made using Date/Time Tool under Quick Tools and a Form Calculation under Widgets.
Check the calculation formula by opening the calculation wizard (click on the wand icon)
Notice the Today date field, Age field and the Days Field are set as hidden (do right click on each field, then hide field).
They were used in the calculation widget.
The form calculates the days by subtracting the present date to the birth date
-Then it divides it by 365.25 (equivalent for 1 year [this shows a more accurate result]).
After that, create a condition that hides the submit button if age is less than 18.
I hope this helps you to sort out what you are looking for. Cheers!