“you can’t make them 18+, they’re a child in canon!” isn’t any different than:
“you can’t make them work at a coffee shop, they already have a job in canon!”
“you can’t make them a human, they’re not a human in canon!”
“you can’t make them evil, they’re nice in canon!”
do these claims sound like utter bullshit? that’s because they are. canon doesn’t have any effect on what i make in fiction