If your looking at just deer the marlin is a great way to go. I know lots of folks don't like the cross bolt safety on the newer guns but it has its uses. It makes unloading the gun a little safer and it useful for treestand hunting.

I know its a unlikely situation but an exposed hammer lever action droped from a distance on its butt is dangerous. The impact can bring the hammer and trigger back and fire the rifle. (had a friend who fell asleep in his treestand.) and worse is the muzzle is pointed up. So the cross bolt has some use in this instance.

The 30/30 is a great cartridge for about 90% of the deer hunting and its a classic as well. The marlins and winnys tend to be slightly smaller in the butt than do most full size bolt guns and that makes them a good choice for am 12 year old.

However, if you reload another bolt or Single shot is a more flexible choice. In most good dual use cartridges like 308, 7-08, 280 etc if you start with a light load using a light for caliber bullet (just the starting load in a manual) you will have a gun with very light recoil and just fine for deer. Once he is a little bigger ( or more of head strong teenager) just switch back to a more normal load and elk is on the menu).

If your 12 year old is a little small for a fullsize gun. You either buy a youth model noew and a fullsize stock later. or buy the fullsize now and a inexpensive youth stock like a ramline (make sure ramline makes a youth before you choose the rifle)