I think it depends on how the younger kids are used in the program. When I was in high school (20 years ago, mind you), there were a few freshmen and sophomores that would suit up with the varsity on Friday nights, just in case. These kids would get their reps on the JV team. Of course, these kids were the kids who matured early, and were big and strong enough to handle playing at the JV/Varsity level.
When I was a sophomore, there was a kid who came in as a freshman at nearly 6'5" and 250-275 lbs. He was on the frosh team about 1 or 2 days before he was moved up to JV/Varsity. He went on to start at OT for NU in the late 90s.
I think the argument for letting the kids play "at their level" is to keep with the social aspect of being with kids their own age and being able to play with their friends.