Is there a forum or category for questions and answers for mature people?

Not around this place.

You will have to make do in whatever category you wish to place it

No there isn't, all categories are open. Besides, limiting a category to "mature" people would be impossible on the internet. If you mean very mature, there is a Senior Category, but even there mature can be hard to find.

All Categories > Society & Culture > Cultures & Groups > Senior Citizens

Since YA is family oriented and users as young as 13 can sign up, you never know what to expect.

But of course there are quite a few senior chatrooms on the net which might be what you are looking for:

http://www.321seniorchat.com/

http://www.seniormag.com/links/chat-forum.htm

http://www.senior-chatroom.com/

http://www.life-over-50.com/

http://www.chatix.co.uk/senior.html

http://www.seniorpassions.com/