There's a tautology encoded in the question. You become an adult when you behave like the people who most people consider to be adults, behave.

What is an adult? Like most words, "adult" encodes a cluster of related behaviors and it's a probabilistic judgment as to whether any individual counts. And it's also shaped by the circumstances of the day. The roles and responsibilities of adulthood change over time, with different social expectations, and those roles may become achievable or less expected to be achievable, depending.

It's unsurprising that the article doesn't really come to any conclusion. The question doesn't admit a hard answer. A better question might be, what is the good life of today, and what transitions and when might make sense in our time.

Our lives are less structured by tradition than times past. But some biological truths can't be denied. A good life, today, might require one to be countercultural, if our ad-ridden culture over-venerates individualism and youth.

I suspect most people only realize these things in retrospect. You don't really know what doors have closed until you find yourself ignored, knocking outside.