We have never requested them in advance but have always had them. When we meet our stateroom host on the first day we normally just go over what we need and there has never been a problem.
As for age our oldest was 5 on our first cruise. We actually brought an airbed just in case the top bunk didn't work out. Turns out we the parents, as usual, were more scared/nervous/freaked out then she was. Then again I was also scared/nervous/freaked out over the balcony (which of course neither of our daughters jumped off either).