I don't think any of the other characters will have aged, I think perhaps we are seeing an alternate universe where Sonic was put into stasis or removed from time between the end of the Genesis era and Sonic Forces sees him return to a Bad Future.
Perhaps Classic Sonic's introduction will be similar to Generations but this time he is pulled into a portal that causes him to skip all of Adventure 1 to Lost World?
Then again, that doesn't explain how Classic and Modern are all of a sudden in this Bad Future (Bad Present?) together. I don't think they will get too technical with the time travel stuff, or if time travel will even be a factor.
I do think that Tails will simply be Tails in appearance, no aging, and that the third character is not tails and is not another Sonic.