This has been a topic near and dear to me for a long time, as I grew up with poor-fitting rifles myself, and thereafter I focused for a lot of years on helping others get properly configured youth rifles for their kids - but it occurred to me several years ago now, I honestly don’t believe the “youth rifle” really deserves to exist. I’ve sat with too many dads when they came back to buy ANOTHER rifle and tried to figure out what to do with their sunk cost in the youth rifle, and with too many kids which have grown up and then lamented having some undersized, under-powered, under-quality, or under-featured rifle, which SHOULD carry all of the sentiment of growing up as a marksman and hunter, but the kids were told it was a “single-serving” rifle and not meant to be a long term part of their life, so they’ve been looking past their first rifle as something disposable…
Alternatively, I’ve come to focus now on sustained value and purpose of the rifles. Purchasing a standard model which will have greater potential for life-long purpose and a replacement stock to be modified for youth LOP, or purchasing an AR-15 with a collapsible stock (or bolt gun which uses the same) offers a youth shooter the opportunity to keep using the same rifle throughout their life, stacking sentimental memories onto the metalwork year after year.
What cartridge? Eh, the choice isn’t so critical. Since most kids should be shooting far more than hunting, and because recoil is always an issue, but also because many low-recoil rounds 1) have sustainable utility throughout a shooter & hunter’s life, 2) share a common boltface with more capable cartridges, and 3) can be found in models which offer modularity and relatively low cost adaptability. Grab a 6mm or 6.5 Creed, 243win, 7-08 etc, and you’ll have access to any short action standard cartridge in the future - and can forever hunt deer and smaller game as well as enjoy low recoil, low cost, high capability pleasure shooting. Many models now offer interchangeable bolt heads and drop in barrels, and some models offer all of their chamberings on a long action, so converting from 223rem to 300 PRC in some models is just a matter of swapping parts around. Of course, AR-15’s offer incredible modularity and interchangeability as well, and starting with a low recoil 223/5.56 or 6.5 Grendel or 6 ARC offers a lot of capability with sustainable purpose.
Walking my talk, my son’s first firearms have been a Marlin 60 and a Savage Mark II, both in 22LR, and for both of which I ordered replacement Boyd’s stocks to cut down to fit his LOP as he grows; an AR-15 with a plinking upper in 223rem and a hunting upper in 6.8 SPC, and a Defiance Deviant bolt action in 6 Dasher, settled into a Manners Gen 2 Compact stock with extremely adaptable LOP. The ONLY firearm he started shooting young which likely will not have sustained utility in his life is my great grandpa’s single shot 410 - for which I found a replacement stock and cut down to 10” LOP until he grows into it - and naturally, passing on a 5 generation firearm just makes a lot of sense. I used it frequently as a bunny gun, even as an adult, so maybe he will as well, but like myself, I expect he’ll spend more time shotgunning with some other shotgun in the future.