Sorry about the age requirements. I'd just like to say that the age range is simply due to putting limits on our research sample. Most studies (
here's a random example) on "healthy adults" put age ranges from 18-40 or 18-50. Younger than that are minors which are often excluded for legal reasons, and older than that are "older adults". When you are studying behavior, biology, etc. you often want to put some broad limits on variance due to age (unless, of course, you are studying the effects of age). If we had an infinite sample we'd love to have people of all ages.
I'm not sure why people place the cutoff where they do - probably a combination of evidence-based behavioral and biological changes and an arbitrary "well why not here" kinda mentality.