Players who are later round picks or undrafted usually take longer to make it to the NHL, for obvious reasons, if they were NHL ready, they'd be high draft picks. Duh.
And players often get PT for a year or two then fade away forever, people here got all excited about Gufstaffson a few years ago, he hasn't been heard from since.
Cam Barker (#3-2004) started at 22 for Q, by 24 his career was effectively over. Guess Q buried him?
Hjalmarsson played at 21 b/c Chicago's depth was brutal, Barker as the #4, and Walker, a replacement player at best, as the other alternative.
Same reason Leddy at 19 played ahead of 38 year old Cullimore and 26 year old Hendry (who played 1 more NHL game after that season).