I think these types of moral questions aren't actually that useful, because the actual problem at the heart of it (and at most things) is the difference in power.

Instead of asking "what age should temporary prostitution be legal," maybe we should ask "why have we concentrated so much excess power in the hands of this one guy who can drop a life changing payment for a one time service and still have plenty left over? Does it really make sense to try to come up with an arbitrary age that we've decided you're immune to coersion?

This entire moral quandary doesn't really exist in the (admittedly idealistic) world where power isn't so unequally distributed.

Regardless, the moral fact is that any man (or parent) willing to participate in this exchange of services should immediately become an organ donor.

If you want to get a better sense of what is reasonable, listen to high school kids talk to each other at the coffee shop, or whatever, and ask yourself if they can reasonably and reliably make informed decisions with long term consequences like this issue would require. (Prediction: It is highly unlikely you will feel that they can.)

For ten million I would let bezos fondle the beans for 30 seconds. Best I can do.