It seems to come and go for me. Much of the time, I still feel that slightly insecure teenager thing.... But other times, not at all.
If I have a house full of little ones... I feel adult. When their parents are here, if they are my age, I don't feel so adult anymore.
When my parents are around I usually don't feel like an adult. Sometimes, but not typically.
When my friends "adult" child is around (someone who doesn't quite have her act together still and is fairly self-involved and unrealistic about life in general), I feel very much like an adult.
I feel very adult around the mom of one of my daughter's friends, in particular. She seems a little contemporary (?) in the way she is raising her daughter, and I am more conservative, and I feel more adult/mature around her. But then there are other parents of other friends, that I feel so teen around.... (sigh)
It just depends upon whom I am around at the time, I guess. If I feel confident in my "authority" I tend to feel more adult. When I don't feel like I am the authority in a given situation, then I feel more teen-like.
When my dad was in ICU, and my mom was sleep deprived and stressed and had no solid answers to straightforward questions (because she didn't think to ask them or didn't want to impose herself on the staff), I felt very adult confronting the duty nurse with my list of questions, and advocating for him. Especially when they moved him to a "standard" room on heart floor, (only bed available) when his issue was his spine. There were specific rules about how to move him in the bed (and NOT move him) and they weren't planning to post it anywhere in plain sight in his room! The very first nurse that came in, I had to stop from causing him further injury, because she wasn't going to log roll him. My mom was visibly bothered by it, but couldn't find the words to say "HEY! HE has a spinal injury, you can't DO THAT." These were cardiac wing nurses accustomed to dealing with cardiac patients. :/ (We made a big sign and posted it above his head/bed on the wall).
Sorry to carry on... but I think for most of us, there are "moments". It is very situational whether you feel like an adult or not.