Oh God, not every player takes a linear path from HS to the Show.
And sometimes a player far exceeds expectations - Trout.
Sometimes a player needs a couple years to adjust, the list is endless.
The point of age isn't that a year is a big deal, rather, it's a datum that helps you judge a player's progress.
Moniak has bulked up substantially from 18-20, that often requires a player make serious adjustments working with a "new" body. But his basic hitting tool was probably based on quick twitch reflexes, eyesight and eye to hand coordination - odds are he still has those skills.
Since he's only 20 in high A, it's far too early to jump to conclusions, the same with Ortiz being 19 in Lakewood. Look at Grullon in his second year at Reading at 22, big jump because he's only a year behind his peers.
I'm less concerned with absolute age than experience, Moniak has a little more than 1000 PA, most hitter's aren't ML ready until they get to the 2,000 - 2,500 range (including college and winter ABs). So if he starts to turn it around the second half this year, he'd be 21 in Reading and 22 in Lehigh, ready to be called up if he's pounding the ball with those 2000+ PA under his belt.
Top hitters in 2015-17 and their first good 120 RC+) ML season:
Trout - 21
Donaldson - 27
Judge - 25
Votto - 25
Altuve - 22
Betts - 22
JD Martinez - 26
Ortiz - 26
Encarnacion - 29
Cabrera - 21
Stanton - 20
Freeman - 21
Ramirez - 23
Pham - 29
N Cruz - 30
Bryant - 23
Goldschmidt - 24
Harper - 19
Lindor - 21
Machado - 23
Arenado - 24
Seager - 22
Dozier - 27
Rendon - 24
Rizzo - 24
Turner - 29
Blackmon - 29
Posey - 23
Springer - 24
Correa - 20
Beltre - 25
Abreu - 27
Zimmerman - 24
Bellinger - 21
There seem to be three general groupings:
Whiz kids 20-22, dominated by LA players and top draft picks
Mainstream 23-26, a lot of college players, but also LA and HS players on a "normal" development path
Late bloomers 27-30, some languished in the minors, others were meh ML players for a few years.
I was surprised how many late bloomers there are, makes you want to take a closer look at Meneses.