I mean, only 5 people get nominated each year in each acting category. There's always going to be actors who get passed over for a myriad of reasons that have nothing to do with age.
There are tons of other equally powerful performances each year that I feel go unacknowledged by the academy, many of them I would argue were better than a lot of the performances in your list.
But that's subjectivity for ya. The Oscars are about 80% politics and 20% actual merit IMO, so it's futile to try and compensate for those who fall through the cracks because the list would then be endless.
Yeah, some deserving kid might lose out because he's a child actor, but another well-deserving actor might go unrecognized because he's black, or Asian, or in a low-budget film, or doesn't have Harvey Weinstein backing them, etc.
Should they each be given their own category?
Oh, one last thing: