Comes in many sizes, shapes, and forms.
Most of our modern societies use a legal definition related to calendar age regardless of the competence, ability, or responsibility of the individual. Other societies use a rite of passage to define adulthood. Sometimes that particular opportunity is generalized and available to all, in others it is limited by gender, status, or to a particular group’s membership.
From there one can branch out to other considerations: religion being a major factor in this arena. when you go back in history – much of the determinations were made related to physiological attributes: when a boy went through puberty and was big enough and strong enough to do a “man’s work” or when a girl passed through menarche so that she would be considered an adult capable of childbearing. Of the three key factors – physical, mental, & emotional maturity – I think that all of us would agree that early to mid-teens do not qualify as adults in most of the current modern societies.
Where am I going with all of this? A good friend (first met in around 1995 if I am not mistaken) finally had a chance as an adult to accomplish her Bat Mitzvah. Health, opportunity, the pandemic I think all played a part in the delay. Her synagogue holds an adult class. This evening, the class (I think I counted nine) combined forces complete with Torah reading. The participants ranged from growing up in an orthodox community in Israel (Bat Mitzvah? Why? Count girls? Why?) through non-observant households while growing up to adult converts.
Flying ½ way across the country wasn’t an option for me. But the pandemic has had some good long term outcomes, one of which is the addition and maintenance of zoom technology so that those of us at a distance can join in.