Ooh, nice detail! I don't have the books in front of me right now either, but I'm pretty sure Sirius refers only to James – that James was 15 at the time. Then again, apparently James has a March birthday, so by OWLs of his 5th year, he was already 16. Ugh, JKR math, whyyyyy?
(Okay, to be fair, Sirius is making an off-handed comment about something that happened decades ago, so it's realistic that he doesn't remember whether James was 15 or 16 at the time. Also, JKR was very much focused on making the point that James was (about) 15 then, and Harry is 15 now, so that's fair enough, too. But still, JKR math...!)
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(Okay, to be fair, Sirius is making an off-handed comment about something that happened decades ago, so it's realistic that he doesn't remember whether James was 15 or 16 at the time. Also, JKR was very much focused on making the point that James was (about) 15 then, and Harry is 15 now, so that's fair enough, too. But still, JKR math...!)