33 years is not that young but not that old too. Most players at this age could play from 2 to 4 additional years without much problems for their performance. Kevin de Bruyne is a good asset for Manchester City, they will not let him go easily even though discussion between them didn't start yet. Kevin de Bruyne will not try his chance elsewhere because most experiences aren't positive at this age. He will try to stay at City even with a lower contract before seeking another club. MLS Inter Miami is interested on him by the way.
33 years is actually old but the players many consider to be the best doesn't want to retire and now they are redefining the age backed of footballers where they get old or become an average player. Many past football players most retire if not all at this age but the importance of players in club has made teams to keep them so the club and the player can remain relevance as long as they like, even KDB I strongly understand his importance in football.
KDB used to be a great asset to Manchester City but this season, I don't think so. He made some contributions but not as quite thoughtful like other season when he was a beast, he literally take Manchester City and Halland on his hand the entire 4 times they won the Premier League and some other leagues but ever since he went to injury last season and came back, things where never the same again and the club isn't going well like back then, I hope he find a better club to get enjoy the rest of his football career.
It's about the individual player and how the can compensate for their advanced age. Some players know how to deal with it, make up for it with positioning and try to solve the challenge of the field in other ways than they used to. But KDB was about top speed and he lacks that now. Or maybe he still has it, but not for 90 minutes. There are other players especially in the midfield that were not as dependent on top speed as KDB is. He has all the other skills too, but he needs those deep runs on the wings and then find his way to the center sometimes. Without that speed, he won't just become a machine distributing passes like Kroos or Modric once did. It is harder for him I think to adjust to the circumstances because he would have to change by too much.