@starwars582 Y/N and Arashi were/are 1A students, so that has them relative to the ages of the others. The high school 15 to 16 range.
In terms of power scaling, Y/N is up there with the top rankers of the world. His current battle with All Might is partly to prove he is more powerful than him. Muteki ranks below both of them.
Unfortunately for Arashi, she has been left behind in terms of power. She's very strong, top of the class, but she’s below all three of them. She won't give up though and will be focused more on in the next arc.
... I'm not saying where All For One stands just yet, but he'll be even more powerful than he was in the anime.