I am creating a membership registration form for our club. One requirement for membership is a minimum age of 18. Am trying to figure out how.

You may ask the submitter her/his birth date and simply calculate the age, show a message based on her/his age.

First, you need to add two date fields, one where you will ask the birth date and a second one where you will capture the current date: 

Then you only need to add the Form Calculation widget to your form and add the calculation to get the age: 

Calculation:

The floor function will help to round the calculated value, find more info about it on this guide: https://www.jotform.com/help/267-Form-Calculation-Math-Function-Reference 

Based on the result given in the calculation you can then show or hide fields in your form, here's a guide where you will find more info about it: https://www.jotform.com/help/316-How-to-Show-or-Hide-Fields-Base-on-User-s-Answer 

I think you could first ask the user's age and then based on the result show the fields so he can continue filling it out, otherwise simply indicate they cannot continue to the form. 

Let us know if you have any question.