Well Larkin came in younger than Mantha. He's almost 2 years younger *and* played a full season before Mantha did. Wheeler came in pretty young as well. And as you acknowledge, cherry picked names.
There are a number of factors I consider. A player who comes in at 18-19 has more time to figure things out *and* they are still developing physically and mentally. ~25ish is a general cutoff point for age in other contexts, like say renting cars. You're still doing a considerable amount of mental development until then and obviously a large part of hockey is mental given its speed and fluidity. But by 25 you're likely at the limits of your development on both fronts. And if you've spent 3 or more years in the league, you've gotten plenty of experience. He's played over 200+ NHL games and a bunch of AHL games. I don't think it's a lack of experience holding him back from his true potential.
Mantha is pretty much 25 now. If he were 22 I wouldn't be saying this. Could he be a late bloomer? I guess he could but I have no reason to expect him to buck the trend. Nothing he's shown me tells me he's going to buck the trend. And even beyond you can look at how rarely a player of his size, at his draft position, puts up 70-80+ points a season. He'd have to defy a lot of odds and given his age, that he's far from a rookie, that he's a forward who tend to develop/peak sooner than d-men/goalies, I just don't expect it to happen.
I think his realistic peak is 75. That's on a good team, playing top line, #1 PP unit, in a career year. That's a damn good player. I think on this team what he's done so far, a 54 point pace over 82 games, is pretty good. Probably on average hits 60-65 on a better team.
I'd love to be wrong. I guess I'll turn it around and ask you why you think it isn't what he is? What makes you think he'll improve significantly over what he's shown us? Besides the few outliers around the league who've done it before.