I have had a Yildiz 20 and 28 bore for both Instruction and the Grandkids use. I borrowed a 'Youth Model' Yildiz in 410 to try for fit for the Grandkids, prior to my buying the 28 bore. All I would say was that I would have to have shortened the stock, even on the Youth model it is 13 1/2" L.O.P. so I bought a second hand standard length 28 bore and had that shortened to 13" with a Kick-Eeze pad. I then bought a dinted wood stock from the Importers in Preston and raised the dent then re-oiled the wood. I now have a short and standard stock for either the 20 or 28 bores (they are the same action). I sold the 20 to a young lady that I was teaching and she wanted to buy the gun, so I now only have the 28 bore. My elder Grandson has been using the 28 with 16 gram cartridge. He is now 13 and has started with a few shots through my 20 bore Beretta, but of course the stock on that is a bit long. He has shot a box of 25 cartridges of 21 gram 7's on 3 occasions and has not had a recoil issue, even with the long stock. He is using 21 grams in the 28 bore too, again without recoil issues. Neither of the 2 Yildiz guns that I have owned have given any trouble and I believe that they are a value for money gun (the side by side being lighter and cheaper). The 20 bore is still going strong and has had several thousand rounds through it now. The 28 has probably had around a thousand rounds and is still a bit tight to open. I do not know which part of the World you are in, but you are welcome to try the 28 bore to get the lad started, if you wish.
