Bought for my 12 year old son. He ranges from average height to slightly taller (larger build but not overweight). He's also a bit taller than average for his age. The shirt is excellent quality but a bit small in length - it only goes up to the belt loops so it rises higher when he raises his arms. It's not that small, it can't be worn, it just won't last many months. I would say if your son is an average weight and height for a 12 year old then Youth XL would be the perfect size. If your child is even thinner, you could probably get even smaller.
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