Its a relatively simple mechanic. There are 28 days in a season so a player who can't gain just over 2.5 sp a day won't maintain his star rating.

A young fast trainer will increase his relative star rating until he's 22 (as long as you don't waste condition) then the decline starts. However its a slow decline initially.

I've been planning to replace a dl/dc for each of the last 3 seasons now but he keeps hanging in there and will start his 9th season as barely 5* (100Q) - but as long as he performs I'll keep him...........unless ofc I make the mistake of allowing 'star blindness' to affect my decision-making

With older players just don't buy them too close to their next star - you don't want to be trying to sell them if they're going to be 6* before the season end.

Longevity or rapid churn or a mix - its up to you to find what way suits you. The only hard n fast rule for me is don't waste your time with crap players whatever age they are.