Include your current age, the age at which that transition happened and (if you want to) describe what happened. It’s ok if you don’t know or it hasn’t happened, yet. It’s also ok if it seems heavy or scary.
I’m 41 and I’m not sure if I’ve always been a grown-up or I’m still not one. My mother has always needed a certain amount of emotional minding, so I could say I became a grown-up at age 14. However, I still can’t seem to clean my house or get my bills paid on time and my 13-year-old and I have a very similar sense of humor.
One thing that sticks out, though, was buying a vacuum with my ex-husband in my early 20s. Probably because it was something for the apartment and not for funsies.