I think it depends on the subject really. I'd never put any of my kids in that early, but it's English and they need as much time as possible to gain and practise the skills they'll need to get a decent grade.
If it's more of a "fact" based subject, where you're essentially just rehashing facts and figures, then it makes more sense.
I know of schools who get younger kids to do GCSEs in pretty crap subjects just to basically get the number of GSCEs they leave with up, regardless of whether the grades will be as good as they would for older pupils.
It's quite common practice as well to stick younger foreign kids in for a GCSE in their native language, so you'll get 12-13 year olds with GCSE Polish or Portuguese for example.
I entered my current Year 11 group for one of the papers in the summer (so they were Year 10). Basically, it was to get one of them out of the way and do the exam while the work for that paper (novel and unseen poetry) was fresh in their minds. It means that we don't have to find time later to review the work, and they can forget about poetry and Of Mice and Men now and focus on the newer work.
But yeah, not a **** ing chance I'd do that with a Year 9 group. For my subject, they just don't have the skill sets at that age.