In my case the next star thing is mainly about 5* players - you're buying a player that is near Scout level - so I'm buying for quality.

The age thing for me is then not about current training rate this season, since its about to hit the next star and the rate will slow (and I don't power train). From my limited knowledge its about the speed that he will get the star back the following season after he's lost that 6th star....and the next etc.