IQ tests are fun. I'm not completely sold on them, but they do measure something. IQ tests, SATs, ACTs, GMATs, LSATs, GREs, etc., basically if you do well on one you generally do well on the others. And I would say fairly consistently. The people I know who did better me on some of them did better than me on pretty much all and the people I know who I did better on some of them I did pretty much better in all.
They're not the end all be all, but it's definitely better to be on the right end of the curve than the left.
But as far as doing well for a 15-year old? Your IQ is your intelligence measured against people your own age so your age is actually irrelevant. You should feel good, though. 138 is a pretty good score.