Hmmm, I guess it depends on what information you are trying to collect.

Are you trying to understand why people buy and/or use helmets?
Are you trying to collect usage based on age?
Do you want information on designing a helmet that would increase usage?
Are you interested in what age groups helmet use might drop off and then increase, again?

As we say, or ask, at work, what is/are the question(s) you are looking for answers to?

All the best, sound like a fun project.