I think we can leave out the generational talents in Ovechkin and Malkin.
E. Staal put up a modest 31 points at 19 in the NHL--Byfield isn't THAT far off at a 21 point pace and heating up. He then spent the lockout year in the AHL and blew up, as many others in that legendary draft class did, at 21. Most Blues fans will agree that Pietro took a bit to get going, he did next to nothing until age 21 and then really took over at 22-24. Brady tkachuk came out throwing but I'd argue he took it to another level this year at age 22 but has another gear left in him, story's not written yet there. Laine's an interesting one, I think he's an exception in some ways because he doesn't care to round out his all around game, he's a pure shooter that hasn't really developed anything beyond that. Rantanen did next to nothing at 19, went half a PPG at 20 and then really emerged at 21, but he's found another gear again in his mid-20s.
@bert covered Barkov, he was going big in his early 20s but he's nuclear in his mid 20s after medicre 19 and 20 year old seasons. I dont' know enough about Trouba's emergence to comment. Assuming you're mentioning Hampus Lindholm--and I'm not sure there's a comparison to be made with the d-men anyway--he really jumped in at 20-21. Ej about the same, 21, 22. Kopitar, of course we're intimately familiar with, was able to score right away, but he absolutely found another gear at 22 and with Terry Murray's development.
I'm sorry but if these are 'counter' examples they're not great ones. The generational talents are just that so of course they burst right in, but the other ones 'contributing' immediately might be generous, and besides that we're not just talking about adjusting and merely playing, but reaching the levels they're capable of. It seems like they start doing stuff at 20-21 like other prospects, really emerge at 22-23, and completely take over at 24+.
That being said I do think Byfield is about a year behind a lot of those guys 1. because he's so raw anyway and 2. the combo of pandemic and broken ankle years sure didn't help, not like E staal's and the other 2003s years in the ahl did anyway.
In the end until someone really grinds out a spreadsheet with the stats this is all just going to be a hypothesis that seems to hold pretty strongly and every time in the past someone has come up with 'counterexamples' they've generally only helped the illustration, so I'm all for keeping it going haha.