Unless he is a real watchnerd at 18, I would either go for a quartz diver at a push.....or more likely a modern-era TAG Heuer (quartz or auto). I got my first "real" watch at 21, which was a TAG Heuer 4000. Nice enough to want to look after it, not so expensive or delicate that you worry about losing it. Still have it, and it was the gateway drug to lots of other more exotic watches.

Even though I love vintage, I reckon most 18 y.o are better off with a modern auqartz or auto....unless as I said at the start, they've already got the bug.

dc