The number of stars means very little. Judge based on their performance. My friend's team has almost all 6 stars and a couple 7 stars but she uses a 3 star striker cause he still scores lots of goals.
Generally if when you buy at guy at 5 stars and he's young, if he plays well for you, as long as you train him enough he'll stay good well into the future. I am level 10 and half my team I've had since level 4-6.

You want to buy players that end in 4 and 9, and likewise, if you're going to sell a guy, sell him while he's still x4 or x9 because he'll get more bids.

Just never assume high rating = good performance, it doesn't, it just shows their talent.