It's already, for the most part, an unspoken rule that most arams are lvl 30 only.

Would it be possible to introduce an option for custom games that would restrict it so that only level 30's can join?

I have nothing against low level players playing Aram, but they are much much more likely to not know how to really play etc...

I think it'd be more of a QoL change that would make lots of people happy.

levels dont matter in aram. im a 16 on my smurf and only join aram matches that are named level 30. eventually someone doesnt see it and starts the game. then when i kick their asses i can laugh. level will NEVER matter on aram, most people on aram that are low level are smurfs anyways. majority of my matches up to 16 were aram against only level 30s that didnt catch it. all arguments are invalid

or you get the lvl 3 viktor who builds black cleaver and is completely confused about the whole game because he can't go back and buy like a normal game.

I remember when I joined an ARAM game at level 23, not knowing the unspoken rule. Man, I never heard the end of the rage. (It certainly didn't help that I got Volibear.)