Somewhere around 25, as freshly graduated bachelor's or master's degree graduates with well-paying jobs after interning and finishing their first years on the job, they truly enter the dating scene as adults with good jobs and strong prospects.

Stop being a responsible adult early enough that you can come hang out at the next meet-up!

Contact the police or an adult protective services agency in her area to do a welfare check on her. 

Maybe have a discussion that you are afraid she can't afford the rent, and you are considering giving her notice because of this (this nips off the "I'm a bit short" excuses)  along what would be necessary for you to be comfortable allowing her to remain as a tenant (catch up on the behind rent, maybe over 3 months period  AND all occupants PASS background check) . 2-3 working adults SHOULD be able to come up with the rent -- I don't imagine that you are gouging anyone.It is possible that if there is a wide open door in front (no 30 notice required if you move in the next 45 days!)

One of our neighbors has a house that is supposed to sleep no more than four adults,  but twice this summer there were 18 to 20 kids in the house and it was booked through Airbnb.

I don't think section 8 pays for adult children ???

@Dave Foster As usual, good news or bad news, Dave has the answer.I'm on my boat right now enjoying my first adult beverage of the evening.  

location=Virmens%27ka%20Street%2C%20Lviv%2C%20Lviv%20Oblast%2C%20Ukraine&adults=4&children=0&infants=0&home_collection=2&s=0Ilm1-ns

I acquired my notary commission by attending a one day course at a local adult continuation school.

Do you typically make all adults (a couple, etc) pay to apply?