I think the whole "mature" thing is just pointing out that Sora will be in his 15-year old form and not in his 14 year old form because they used his young form excessively in the last spin off titles.
They want to emphasize that this is indeed the sequel of KH II (and DDD).
they won't age sora because the KH style is based on these kids and turning them into adults would be too close to Final fantasy titles and won't work with the playful disney style. Therefore they won't make the main cast any older.
(if you are wondering why they did with khii anyway- sora was sleeping for a year!)
Sora will get a new outfit, though. And hopefully old and new drive formes.
And as they just used a placeholder image in the trailer status bar they sure will use 2D images as soon as the new artwork of 15 year old (!) sora is released.
They also emphasize that Sora will be the Only main character and Goofy and Donald will be his allies.
There will be allies depending on the world and maybe others playable for a short time (such as King Mickey, Riku in KHII) such as Riku, Kairi, and King Mickey.
summons will probably enable Sora to turn into/ summon Roxas Xion and other connected hearts.
i bet this will feel really close to kh and khii and will merely incoporate some elements of spin offs such as limited command deck functions and some flowmotion action.
and this is what fans actually want, is it?