I've been hearing different things on the subject.

Are "youth stocks" readily available for rifles such as bolt action .243 or .270?

I'm thinking about a rifle like that for an 11 year old kid and just wanted to swap out stocks til the hunter was older and bigger.

Any rifle manufacturer in particular?

Winchester
Savage
Remington

Unless the youth hunter/shooter is small for their age I think it is a waste. Myself, my brother and all of my cousins and friends started hunting with full size bolt actions in rem ruger Winchester and none of us had a problem.

At 9 years old I had absolutely zero problems shooting a full size Remington 700 bdl 270 and I was not a big kid for my age.

Every kid is different, remmington has the model 7 youth and howa 1500 in 7mm-08 is fine also my wife is short and the howa and model 7
fits her just fine.

I have a Blackhawk Axiom stock that I put on my kids Weatherby Vanguard S2 - 243. They make different versions for different manufacturers and calibers. The adjustable stock makes it nice for my 7 year old and my 9 year old that are different sizes.

Many manufacturers make youth stocks. They are 2" shorter than an adult stock, making them 11 1/2" trigger to recoil pad (length of pull). I'm a proponent of youth stocks, they help the child manage recoil better, and if they don't manage recoil they won't want to practice shooting, and if they don't practice shooting they won't make a good shot on an animal. Then you have hurt feelings. Same holds true for adults.

My son was 8 last year when he started shooting. He hunted last year with a full size rifle and it didn't fit him. For Christmas, I bought him an older Remington 700 and cut the stock down and refinished it before giving it to him. It fits him perfectly now that he's 9. We just went shooting last weekend and it was much easier for him to shoulder the rifle properly. It will be easy enough to replace it with a full size stock when he's ready. I'm not a fan of any of the youth rifles, typically they are lesser quality synthetic rifles. IMO, your better served to buy a wood stocked rifle and have it cut to fit, then buy a full size stock to fit when he's ready.