My DS is nearly 10m and has just started doing it today (v excited!) Until now he has shown no interest whatsoever in crawling/bum shuffling/anything other than sitting like the King of all he surveys so I guess when they decide they want to do it they just do it! It didn't cross my mind to worry about it, I think they just all go at their own wee paces.
Well this is really variable as you've probably found out by now. DS1 could do it at 8.5 months. DS2 only did it about 2 days after his first birthday so it's massively different from child to child. I would say DS1 was probably fairly early doing it whereas DS2 was towards the later end of the scale but either way they'll do it when they're ready!
DS2 is 11.5 months and still can't do it. He is a bum-shuffler though and has always hated being on his back or front with a passion. He only discovered bum-shuffling at 9 months and rolling both ways at 10 months. He seems to do everything in his own time. DS1 I can't remember at all whoops.
I can't remember how old DS was, was but I do remember that it was really sudden. I had no idea he was close to being able to do it and had never seen him try, then one day he just could! Weird, because so many other things you see them trying for ages. I think if they can roll and sit, it's more about them realising how to do it than needing to develop physically.
About 8 months for all four of em. The first three all walked independently at 10 months though, the fourth at 12 months on the nose (although she cruised from 9 mo), so I think they might be more physical than the average.