I don't think it is as much of his age (21) as it is experience (barely in his second year). One would have to believe that McD wants Edmunds to cover the middle/slot, stop the run, and rush the passer on occasion. Right now, Edmunds is good at covering the slot receiver and TE. He looks to need more work with stopping the run; if the DL doesn't hit the back first, there is a good chance the RB is making it to the secondary for a big gain. I haven't seen him rush the passer much, though that is probably more a function of scheme - Frazier doesn't blitz a whole lot.
I was also hoping (expecting) that Edmunds would have bulked up a little this past off season. Doesn't look like he has done this.
If Edmunds is still struggling against the run and shedding blockers by season's end, I'm in the camp that he should be moved to OLB and Beane/McD should look for another MLB - either FA or draft.