I think it's more a warning to be careful & considerate of the factors & consequences, not a 'given'. Not something to be advised, that a beginner buy a young/green horse, because riding on a large, live animal is potentially dangerous enough without adding lack of experience on both sides to the equation. But if you're in that situation, just be aware, take precautions & take it slow & ensure you have a good trainer/instructor to help & teach you, and someone experienced(maybe the same person) to help ride & teach the horse, when needed.
I am quite an experienced horse person, but when I went looking for a first pony for my kids, wanted something 'tried & tested'(not that I was going to be an overprotective mum... until I considered my kids on horses!). But after looking at & turning down many 'bombproof' and experienced 'kids ponies' who didn't feel right, I ended up getting an 11.2hh welsh mountain boy of 7yo... who had been left alone with no handling or care since he was a yearling. The kind of horse I STRONGLY advise a beginner(or parent) doesn't even look at. The current owner only took him, knowing she couldn't keep him, to give him a chance, as he was 'free to a good home' or going to the doggers. She had handled him at home a bit, given him a couple of weeks with a pro trainer, to be started very lightly under saddle, but that was all.
When I brought him home, I found he was terrified of little, erratic people, among other things. Also found he was seriously terrified of any tack aside from halter & lead - testament to little, rushed training I suppose. My daughters were 2yo & 3.5yo when I got him. So I did put in quite a lot of time in further training, getting him used to the kids, taking him out & about with my horse & riding him(I'm too big, but for very light riding...) before the kids were allowed to even be on the ground near him. He was only a lead pony for a couple of years, & was fantastic for that, but I worried how he'd go when I gave my kids 'free rein' as he was very forward and playful. I ensured they had an independent seat, practiced 'emergency stops' until they were blue in the face, learned to control him while I had the lead(as just as a 'safety chain'), before they finally started riding off lead. He has been the BESTEST little pony! From the first time I unclipped the lead, he put his head down, stuck his top lip out in concentration & went so slowly & carefully! He really babied my kids along & helped them gain confidence! He's taken them all over the place, around town, in the bush, through rivers, in traffic, jumping, swimming... standing on him!
My eldest grew out of the little tacker then had to ride my big guy(so I had to walk!), until a few years ago when a 13.2hh brumby (hafflinger type) happened upon us. He was about 5yo at the time, handled a little on the ground only, owned by a client of mine, who decided she didn't have the time to put into him. Again, I started him myself. And my elder daughter was a reasonable rider by then, but nowhere near what I'd call 'experienced' for a very green horse. I figured I'd be happy to have him going well & she could ride him on lead within a couple of months... it was winter & I also didn't have much time to put in then. But I reckon he must have been 'born broke' - he seemed to learn everything well in only one lesson, nothing fazed him, I was on his back within the week and my daughter was within a few. I've hardly got a look-in since! She has taught him herself to jump, to neckrein, and all sorts of tricks - including 'roman riding' - riding standing on his back. He's been dressed as a reindeer at xmas, mobbed by kids at the school(I of course supervised carefully), balloons tied to him, flags carried, whips cracked, he's towed kids on skates...
We started them both in a cart a couple of years ago - something I'd never done before. And about then, kids wanted to do formal ponyclub, and they were both good riders with 'soft hands' by then, so we started the ponies in bits for the first time, because the PC rules stipulated it. They'd only been ridden in halters until then.