On the last IQ test that I took, I scored 147. The test was administered through a Columbia University affiliate.
With that being said, I don't think IQ tests are very reliable, when attempting to measure one's intelligence. In my experience, I have known several bright individuals, who are very intelligent, but didn't do particularly well on their IQ test. So, I wouldn't give the test more merit than it deserves, when it comes to judging intelligence levels.
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.
I took an online one a long time ago and got a 151. Also about 5 years ago my gf of the time (now my wife) told someone else I wasn't a genius. So I went and took a MENSA test at Cal Tech and got in (pretty sure they only take 130 IQ and above), and now I like to throw it in her face (jokingly, of course). I never went to any of the MENSA events. I think they met at bars and restaurants and had scrabble tournaments and crap like that.
Anyways, all IQ tests really measure is how well you do on IQ tests.
IMO their not that accurate I have taken two. One said I had a 129 and the other said a 150. So I have no idea which is right. I also agree with Karoz. I know very smart people that didn't supposedly do well on a IQ test.