What is JotForm?
JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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It is possible to calculate the age of the user inside of your form.
You need to add two date fields, one of them with default date of today.
Then you need to add form calculation widget and set condition inside of it.
Here is a demo form I made you can test it and clone it.
Hope this will help. Let us know if you need further assistance.