I think you can suit yourself really.
In the Supergirl movie Lucy is in school with Linda, and supposed to be about 16 I think, when Lois and Clark are already working together.
In comics she was usually shown as just a little younger than Lois and in moderately responsible jobs - air hostess, later air traffic controller - and sometimes linked romantically with Jimmy Olsen.
The L&C version seems to be about five years younger, but that would still make her 20+ since Lois has been a journalist at least six or seven years.
