It's very rare , but not unthinkable.....
I know an 18 yr old who is NW5, and will be NW7 in 1 more year, 2 if he's lucky. But this guy you saw.... was he bald ALL over his head (including the sides and back) or did the sides and back have some stubble? NW7 is severe male pattern baldness, but even in NW7s the sides and back retain a significant amount of hair. If he lost even that DHT-resistant hair, it's quite likely something other than (or in addition to) male pattern baldness was affecting him.
I have seen some guys in their mid-to-late 20's who are quite bald
(NW5+) but its VERY rare to see a teenager so afflicted. Sure, I have seen many teenagers who are receding (NW2ish) but otherwise have OK hair. But this extremely early-onset and severe male pattern baldness often runs in families and afflicts nearly everybody in that family. One of my distant uncles is in his late 30's and NW7 (with rather thick hair on the sides and back), but he did'nt lose his hair recently, he's been bald atleast since my days as a toddler (I'm going to be 18 in a few months) probably before that. So he also likely lost his hair very early, in his teens or early 20's.
Seriously, it is MUCH worse for these extremely early balders. They can't even enjoy their hair for five years past puberty (around the time most people start to care more about their hair;children don't usually care much) But ironically, it is often these early balders who accept their situation much more easily than the typical balder who starts thinning slowly in his late 20's.
Cheers,
Arjun