seems pretty clear:

main booking search asks # of adults (age 15+), # of children (age under 15).

https://faq.flyfrontier.com/help/can...d-travel-alone says:

Any child under the age of 15 must be on the same reservation as an accompanying adult

I would assume rules differ for any international travel.

also keep in mind frontier won't give hotel vouchers even if they strand them during a connection or divert.... and hotels won't check in minors without an adult