The exact age difference is never revealed, but I'm guessing at least three or four years, probably more. Yeah it is pretty significant, now that I think about it. If there was less of an age difference or none at all then maybe I guess they could date? Bree doesn't like Bob back though, so maybe not.
Edit: Bree is 18 in Bionic Island, and as a guess I would say Bob is about 14. Bob and Spin are the youngest of the soldiers, but Spin I think is maybe 13. Never specifies in the show.