Of course, they could use make up to age the entire cast 10 years, but...why? Just to make this movie right this very minute? That doesn't really make sense to me. The movie will probably be a much bigger deal as nostalgia if we see the cast returning to their roles after 25 years or whatever it would be. And they will look like whatever they look like. If a 40-something Daniel Radcliffe looks the same as a teen or early 20-something Daniel Radcliffe (highly unlikely), then Harry Potter just has really good genes, I guess.
Trans issues may not be a big deal for the original Harry Potter fans, now in their 40's, who are, I suppose, the intended audience here, but it's pretty much ALL I hear about JK Rowling now among younger people. It's sparked a lot of conversations online about separating the work from the artist as well as the incongruity of the creator of the Harry Potter universe (the stories are largely about average people rising up against fascism) being so upfront about her transphobia. There's a disconnect there that mystifies and disappoints a lot of people.