I've come to my Zed from a 93 too!
With the booster seat he's the same height, roughly, as an adult would be and the passenger seat is pushed back slightly. (As long as the seat belt fits correctly over the shoulder, he's fine) I was always impressed with how strong the Zed feels over other cars of a similar style, but also have no issues if he was there in an accident and the airbag went off. Certainly no different to him being in the front of the 93
I'm in a similar situation in that I only have the kids at weekends, but if I have one of them on their own, we always go out in the Zed
Edit: I think the reason people talk about kids being 12 is to do with height / size and making sure the seat belts are safe and not around their necks, which is where the booster seat comes in.