To answer your second question first, this subforum and the general area should do fine for questions for now. If it does become obvious with more traffic and posts relating to this then we could probably open up something like that, but for now, I think this area is alright.
As for RPing your age, that is feasable. But you have to be careful. Since you are RPing as your younger OC, anyone else you RP with would have to be be made aware of this (probably in the begining of the topic or title). Otherwise when you RP your character again when they are at their 'normal age' if you ran into those characters again or anyone they knew, they would have to account for all that time that has passed since. I doubt anyone wants to suddenly have to age their character 10 years or so or assume that much time when by since they last had anything to do with you. But so long as you inform any participants of this, I think it would be fine.
If you take this to the extreme though, like say a 100 or some years and further back, you would be better of making an AU (alternate universe) topic in the appropriate subforum.