When I was like 15 or 16 I got in to PE, sadly I didn’t stick with it more than 6 months, but I saw gains and I had no issues. I only recently (3 months ago) got back in to it as I was getting rather depressed about the loss of what I gained. SO, to answer your question I really don’t think age is a factor. In fact, back in the day I always heard that doing it while you’re naturally developing (a.k.a. Your teenage years) can only help you gain. Is that a fact, I don’t know and most likely it isn’t, but whatever.
I had no issues then, gaining about an inch over 6 months, and I’ve had no issues now gaining an inch in just under 3 months. Don’t let the wording fool you though, I strongly believe my past PE experiences helped in that fast gain basically because I shot back up to my “old” length rather quick — talking the first month — and I’ve really only gained about 3/10 of an inch in the last two which is more usual.
P.S. - I’m 22 now if you were wondering