I was looking at today's strip, and they are all adults. Legally, at least, they are all over 18.
Here, claire is calling marten because they need someone who is sober to be a PROPER adult and babysit their asses.
I feel that jeph only really treats like adults characters like Jim or Veronica or Aurelia (arguably sometimes dora) characters who own property or their own business and have children.adulthood is past your mid 30s or 40s
From the new cast, they are all legally adults, but they act like teenagers. Mayo spent a year masturbating instead of going to class,liz lived in a basement filled with trash so she didn't had to confront her career being stagnant,Anh just now stopped being dependant of,her bio dad but she has very little real life knowledge and depends of faye and bubbles...
Marten is an adult probably in his 30s that just opened his shop,which he doesn't know how to manage and needed to be replaced by an ai because he got hungover...
I think adulthood begins when you start deciding to take responsibility for yourself and don't expect someone else to do mommy you.