Why are people setting timelines? No player is the same. What does it matter if they are 21, 22, 23? Johnsson is 23 and look how far he has come. Montour only made the NHL at 23 and he is a good promising D.
I think he needs one full year, minimum. He needs to become the 31 D on the Marlies. He needs top PP time, PK time, and shutdown minutes. Really stress test him and pull back when he gets overwhelmed. Let him get stronger. If he makes that step this year, then so be it. If he doesn't, give him his 20 year old year.
Eventually he will have an awesome season. When he does, doing the above, send him back down AGAIN. Let him over ripen. Give him 30-40 games to show last year wasn't a fluke and then bring him up.
Biggest difference between us then and now was we would rush him to the NHL this year. The team is good, he would be a bottom pairing guy at best. He isn't a huge upgrade on Carrick at this time.
Let him develop.