Adulting is actually not a process that needs to be linked to an age group. If you tie it down to that, you limit the process.
There is always another career to pursue. Another adventure to face.
Adulting is "hard", because it is a learning process.
And whoever stops learning and exploring, breaks with a constant process of "becoming of age". It is always terrible to see this on others. Giving up the "beginner mind", giving in to a routine which is supposed to shape the rest of your life.
Adulting can become a wonderful habit.
I know an old lady. 85. Still driving around in her little car. Still taking care of herself. Has been through many things. Almost died of tuberculosis after the war. Became a journalist. A music teacher. Walked the red carpet in Cannes once. Became a director. Retired. Started a special kind of art photography. Became a practicing Buddhist.
Still an adventurer.
Learned to use a smart phone a year ago and complains about difficulties to load her smartphone-photos onto her computer.

At peace with herself, she still faces many little adventures.
Until eventually she will encounter the ultimate one.