Avon is 20 and Desnoyers is 21, this is where physical maturity matters, if you're in a scoring role on a top 6 line you can get away with being smaller if you're very skilled (TK, Brink), but in a bottom six role you have to be able to hold your own in the corners.
It takes time for players to fill out and add strength, even with good training programs (just look at Eagle players from programs like Georgia and Alabama, at age 22-23 they can still add strength), and most players don't get as good of physical training in the CHL or SHL like they do in the NHL (quality of facilities, size of training staff, willingness to subsidy off-season training).
One reason few players improve a lot after 24 or so is they're physically mature and have substantial on ice experience, so there are fewer avenues for improvement, usually more a matter of tweaks and opportunity (right place at the right time). Physically, the biggest changes are probably between 18 (the draft) and 22-23. Players usually add 15-25 lbs to their draft day weight, depending on genetics.