I don't have a problem with ages but if the character in underage, it needs to stay clean, not get sexual.
Example here:
Not okay unless the story is about a boy being rescued from his home where he was being sexually abused. But making it look okay is not okay.
Oh yeah if the author wishes to have the minor have their diaper changed, leave out the rubbing cream on the pussy or the penis, the arousing, etc. "She took off his diaper and rubbed some cream down there." That is fine since it's not sexual. "Then she took out a diaper and placed it under him and pulled it up between his legs and taped it shut." That is also fine too since it's not sexual. If a kid wrote it, that be a different story but it still makes me uncomfortable when I read that part. It's a good away to drive away readers.