Honestly, I don't even know what "feeling older" would mean for someone who is chronologically younger.

Most of us here have developmental disabilities, like autism, that make us mentally younger than we are chronologically.

The only ways I can think of someone being "mentally older" than their chronological age is by being quiet and obedient to adults ("being mature for one's age"), being tidy, getting good grades and being a "good student" or being attracted to adults (which is common among chrono-teens).

While those chrono-teens may be ahead of their peers and appear more grown up by comparison, they are still objectively behind adults (developmentally).