My son is almost 3 yrs old but small for his age as he is under 30 lbs, I purchased the 2-3 size and it fit perfectly and I would say then that these run small so you might want to size up based on weight. The undershirts seem of good quality, my son wears them daily to absorb sweat during the warmer temps. I've washed them several times and have had no issues and I should mention that I let them air dry. Great price point as well compared to the better known brands.