I don't think this holds any water. Plenty of orphan children. Also, my parents are alive and I firmly count myself in the adult camp.

Like the article, I think much of what makes you an adult is taking responsibility. For some, the first time that happens may be when their parents die I suppose.

By this metric my uncle, who died in his 70s, never became an adult. My grandmother lived to 93.

When a relative in the generation before me lost the last person in the generation before them, they surprised me by saying, 'I'm all alone now; it's just me'. There were no parents, aunts/uncles, etc. to advise them, support them, guide them. They faced the wilderness on their own.

Another way of looking at it is, when you switch roles with your parents, and you become their caregiver, their source of strength, their guide through the world. When they can no longer help you. One example is the Godfather, when Al Pacino's character starts caring for Marlon Brando's.

(Other commenters are taking this idea far too literally, looking for exceptions.)

Not a happy definition, but at least a clear and consistent one.