I looking on-line for possible pensions in Paris, I noticed in the fine print for some of them the caveat "age-restricted", meaning, I presume 30 and under only. Do the pensions really apply this?
The hostels that I have come across having an age limit are not the HI hostels but rather certain private, independent hostels with the age range from 18-35. They advertise as such. Most private independent hostels do not have an age limit. The last time I stayed at a private hostel a couple of years ago there are those one half to one third my age, also in a dorm room. No idea as to how strictly this age limit is enforced. Absolutely no age limit for staying at a Pension.
Some cater to students, others accept families and seniors. I'd expect to abide by the rules, because some hostels check ID's.