i think i have good news for you woodfly...
...my school photograph from when i was 15 shows are very distinct expression line on my forehead. my mother has many expression lines on her forehead, and i always made similar expressions with my face. i was on the field hockey team and always in the sun with a mild sunscreen on. i stopped making expressions with my forehead and starting wearing a good high spf sunscreen everyday.
...i am 25 now and there is no remnant of that one line on my forehead that was so visable in that photograph 10 years ago. i have a perfectly clear forehead with not a single expression line. i don't know how it happened, but since your skin is young and has constant cell turnover, i think it could be possible for it to go away. this is just my experience, but mine really did go away. of course, expressions help tell a story of a person, so i don't know if you want to cut out all the expression of your face to get rid of a tiny line. but maybe be more conscious of the expressions you make.