Hollywood being Hollywood, they could easily cast a late 20's, early 30's ATJ to play teenage/early 20's Illyana. They would have to cast her in the very first X-movie though, which is more possible than people think- she fills a niche no one else can, plus her personality would be different enough from the rest of the presumed cast (Cyclops, Storm, Beast, and if she's there, probably Colossus, and maybe Ms. Marvel) that she would stand out. Plus, she has spin-off potential for a movie or Disney + show. Although is more likely they would simply get a new actor.
That said, RDJ, Ruffalo, etc, being older doesn't matter, because we're talking about very different characters, and I also don't get the feeling that at the beginning they expected the MCU to run for as long as it will. Plus, neither Iron Man nor Hulk are that demanding physically.