I think it’s kinda like the old dating age formula; you can date people (your age) / 2 + 7 years old, and you feel like that’s the age of an adult.
When I was 15 I felt like ann adult, but people younger than me were teens. When I was 25 I felt like an adult but people under the age of like 20 were just kids. Now I feel like people in their early/mid-20s are just about adults. I’m sure when I’m 50 I’ll think back to myself now and consider myself barely an adult.