I have had my share of assumptions made about me based on my age, but this isn't one of them. a lot of people are rude and assume you won't argue with them, regardless of your age. But also a lot of people for some reason can't read their boarding passes or the signs in each row, and can't find the right seat, so sometimes there is a more innocent explanation. finally, the first thing I do rather than notify a FA is double check my boarding pass. even as a frequent traveler I have sometimes made a mistake as to what seat I'm in.

(Also people constantly cut in line. I haven't seen this change between my early 20s and my early (ok, almost mid) 30s.)