How old are you, Jack?? If the answer is “older than 16,” let me be (apparently) the first one to tell you that it’s creepy to mess around with 16 year olds at practically any other age, full stop. I’m sure you’re capable of reciting consent laws from memory, but before you respond with that, know that there’s really nothing you can say that will make me think it’s all right for a 21 year old to be put together (especially by force) with a teenager.
21 and 16 are just numbers. Of course lines must be drawn and they (age of consent, coming of age) vary from country to country and it's perfectly ok, everyone think that a certain line is better than the others and they apply it. But there are things that are more important than an arbitrary line, Sanji is not an adult because he is 21, Pudding is not a child because she is 16.
It's much more fluid than that and it really depends on the individual cases.
Pudding wasn't certainly an innocent child and she didn't even look like one