Would your answer change if she was actually 18? It still seems crazy manipulative. In some countries, the age of consent is 16. Would this be okay if she's 16 in one of those countries? (Let's ignore countries below that age)
I struggle between two ideas: One, where I believe that at the age of majority, a person should get full rights (voting, emancipation, legal, consent, medical, etc.) and it seems wrong to let people vote but not make choices about their body (like drinking alcohol). And two, protecting the young from themselves, like by restricting labor, or setting smoking and drinking ages higher than a majority age,, because those damage still-developing brains way more.
We can fight about what the age of majority should be (16, 17, 18, 21?). I would definitely be okay if this tweet was about a 30-year-old, but I'm not okay with it being a 10-year-old. But whether it's 16 or 18 or 22 where it crosses the line is tough for me.
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.
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.)