A baseball season is a baseball season. 81 games are not necessarily 162 games divided by 2. What I’m getting at here is that just because a guy has one bad half and one great half, to combine for a decent season, you can’t expect him to be great the next year. You can’t expect him to be bad either, you expect him to be nothing other than decent. Some guys hit well later in the season after they have a bunch of ABs behind them, some guys tire halfway through the season and decline. There are other factors, too. If someone wants to trade you a Texas Ranger hitter in a fantasy league because they didn’t play well in April and May, you should seriously consider it. Because in Texas, the ball takes off in the heat of the summer. All kinds of things factor into streakiness, or even several good months of play. Anyway, when you isolate July 14th through September 3rd and say someone has gone 6-0 with a 2.42 ERA or whatever, you have to remember this doesn’t exist in a vacuum.
Now that we’ve got all of the caveats out of the way - with young players, there is more of a glimmer of hope than just external factors. As they get older they get better. A full season, or a few full seasons in the majors leads to improvement. That improvement may be visible in March, it may be visible in May, or it may be visible in September. And some of these youngsters may have figured it out, but had tired legs in August and September. So using some arbitrary endpoints (a term I believe I first heard from Keith Law), here are some of them with a few players you may have heard of.