Generally, most homebrew stores don't require you to be 21 to purchase supplies. Its kind of a gray zone, since you are really not purchasing alcohol. State laws might be different, and obviously homebrew stores will enforce it differently. Most of the products you purchase are considered food items. Malted grain, hops etc. Also, there isn't really anything illegal about an 18 year old purchasing buckets, hoses etc.
Although its technically legal to purchase the supplies, I'm pretty sure its illegal for minors to posess and consume alcohol. State law also varies on whether minors are allowed to consume alcohol in their parents home.
I guess it all comes down to your view on it. Not sure whether you are parent, aunt/uncle or an actual smart minor that didn't put down their real age. If the minor in question is genuinely interested in brewing, the process, flavors etc and being responsible its one thing. If they want to make prison hooch, thats a whole different story. I'm 24 now, but I started brewing when I was 19. We couldn't buy beer, and its really hard to get someone to go into a big beer store and pick out things for you to try when you can't look at them. We wanted good craft beer, so our only option was to brew. We bought the equipment online, then later found a store in our city. They never once carded us.