Watching your team's prospects as they come up, wondering whether they are going to make it in the pros - or whether the players who made the team are ever going to be stars, there are two main factors to consider. Age and injury.
Injury is obvious. Injured players don't perform. I mention it because so many people leave it out of their calculations when projecting.
We used to say that baseball players improve until they are 26-28 years old. Some sabermetricians would shade that to 27-29 on the basis of newer statistics which they prefer. I disagree, but fine. At about age 26 or 27, BA starts edging down and power edges up. Players often maintain something near their peak value until age thirty. Most are gone by 33, certainly by 35.
Look up your favorite players from the past if you doubt this. The best players can often sustain a high level for many years, but even allowing for this, check out which year Nolan Ryan got his 383 K's, for example. Leave modern players out for the moment. Just browse around in the old days long enough to establish that even in the pitcher's era 1963-68, a hitter might have is best year in that age range.
I learned this from Bill James, who I admire as an explainer of statistics, an evaluator of talent, and a writer. But he knew the statistics well enough to know that steroids were changing the game, and he mentioned barely a hint of it. That is moral cowardice.
There are exceptions to the 27 +/- tendency, of course, but these are minor. And they are usually gradual, revealing a pitcher who is learning his craft, for example, or a hitter learning to be more selective and draw walks under a particular batting coach.
In the modern era, there are many exceptions, and we can guess why. It is tough to know for certain with many players, because the change is gradual, or because it involves an excellent player hanging on at a higher level than one might expect.
Armed with this knowledge, consider the statistical jump, at this age, for this player. A 200-point rise in OPS at age 29, and another 200-point increase at 30.
It just doesn't happen.