Honestly, Axel's physical age could be anything from 15-25, we don't know... Although he obviously doesn't still look 15 so his physical age is probably higher, we also don't know when he became a Somebody again, so as the Lea we saw in the trailer, it's possible he's aged some more (we don't know how much time passes between KH2 and DDD)
From the end of KH2 (Sora and Riku return home), then happens CODED, then KH2 ending (Sora, Riku, Kairi, reading letter), then finally the beginning of DDD
Literally not a lot of time lapses. Riku just cuts his hair before they go to see Yen Sid.