I'm 29 and have two plaits occasionally for work, more often one. I do think one is more professional but I sometimes want a change. I might also add ribbons on the end.
I would have thought in a charity shop it would be fine.
On a side note: How do you get a bun that's comfy and stays put all day? I have similar hair to you, thick and long, and I can't make it stay in place without about 3 scrunchies.