Najee Harris peaked athletically in high school. He looked like the next Adrian Peterson in high school. But instead of getting bigger AND faster in college like Peterson did, he got bigger and slower. He’s a good RB with an NFL career ahead of him but he’s not special. He’s basically a great looking ILB playing RB.
In comparison, our young RB’s have more speed which means more upside to break off explosive runs. Explosive runs are what separate the good RB’s from the great ones.
Athletes peak in high school all the time. Individual humans age differently. Some guys have a biological age that results in them reaching their athletic peak at 17-18 years old and then it’s down hill after that. Others have a biological age that doesn’t allow them to reach their peak until they’re in their mid to late 20’s.