My son, a skinny 12 yr old uses a 357 Max (with 357 Mag or Max loads) and shoots it well. I tried him on a 30/30 a couple of years ago with factory loads, but he wasn't happy with the recoil. I was going to build some lighter recoiling loads, but ended up with a 357 Mag Handi for him, and now he's got the 357 Maximum. He used the Youth stock on the Mag for awhile, but now is grown enough that he's got a Survivor stock on the Max (its longer than the Youth, but not as long as the Adult).
Everyone is different, but I developed a flinch at 9 yrs old after shooting my dad's Model 94 30/30 and it took me years to get over it. I wanted to be very careful not to induce the same flinching in my kids, so as soon as they decided the 30/30 was too much for them, I went to the 357.
Not to say we won't go up a notch (to the 30/30) when it is appropriate - just that he's now shooting actual 357 Maximum hunting power loads, when last year he was shooting 38 spls, and moderate 357 Magnum loads.