We took the leap last year and got DD her first pony at 10. It has been the making of her. We have a 13hh Welsh C who was not quite 6 when we got her, so we didn't follow all of the wise advice we had from lovely people here and elsewhere, but it has worked out brilliantly for us. It has brought DD's riding on tremendously and they have a brilliant partnership, if still a little scrappy at times!
What I would say is the younger ponies need not just a competent rider, but a fairly mature head on their rider....DD at 10 has struggled a little sometimes with how to feel about and react to feisty young pony's behavioural quirks and 'learning curve'. She's now 11 and does better with it, but early on, she would get upset and frustrated at pony not getting something perfectly first time, and we had to have a lot of discussion about DD's job as a teacher for the pony, as well as just riding for fun. She now takes it really seriously and does a fab job....a couple of years ago she just wasn't ready for that responsibility and would have only thrived with an easier pony life, as it were.
We haven't once looked back or regretted our decision though. It can work out with a younger forward pony, but you do have to go into it with your eyes open. We too are well supported at our yard, and I have ridden all my life and loaned on and off, but this was still a bit of a leap for us, we were just incredibly lucky that it paid off. On the flip side, there is a 9 yr old girl at our yard who was bought a pony a couple of months ago and barely rides it (her non-horsey parents still pay for her to have lessons on a riding school pony) because it is just too much for her. It is so sad to see.
As a final thought..., £6k is a LOT for a 12hh Welshie that's a young project pony! We paid just under half that, for more or less the same, although she's a bit bigger at 13hh, just under a year ago (so was already lockdown type price era).