My son is 84 kg, 9 years old and about 50 inches tall. Last summer he wore a 10/12 bottom and a medium/large top. I ordered XL because I think his waist measurement is a bit larger than average and/or boys underwear in general is just weird, but I digress. XL seemed to fit, but not for long. I can't tell if it was because my son had gained a few pounds, or his lower body had shrunk a bit, or both. This will definitely hold up for many washes (in cold weather) and I tumble dry our clothes. I am happy with how they have held up, not stretched or come apart at the seams.
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