To further spell that out. Acuna has a 60/55 speed rating. Meaning his current 60 speed is quite accurate and as you stated will likely slow down as most prospects do.
Except that he's so young he's not going to be losing much speed in the next four or five years, unless he really fills out. Obviously at some point everyone slows down, but at 19 I think that's quite a ways off for Acuna. I don't see him as the type that's going to pack in a bunch of bad weight.