I agree with dude and CJ - it sounds a bit odd to me, too.
And another (minor) point is that in the UK (where the scouting movement started) Australia, New Zealand etc. the girls are "(girl) guides" and not "(girl) scouts".http://www.girlguiding.org.uk/http://www.guidesaus.org.au/page.php?pageid=127http://www.girlguidingnz.org.nz/
So it also depends where you need it for - although it is not really a problem because both are understood (the World Association of Girl Guides and Girl Scouts).
And what exactly do you mean by "old"
Do you mean she has been active in the movement for a long time and still is?
Or do you mean former or ex-guide/scout? (Although it is said "Once a Girl Guide, always a Girl Guide." :-)
But I don't think I'd say "merely
particpating" in scouting/guiding - it sounds a bit unenthusiastic.
"She has been/was an active girl guide/scout for many years, working her way up to leadership in her group."
If you want somethng less prosaic, I suppose you could use something like
"She is an old hand in the girl guides/scouts ..."