1 month ago, I bought a 3T for my almost 3 years old son whose height is at 65th percentile but on the slimmer side. It fits perfectly and still has room to grow. I expect he can wear it again this fall and winter.
It's slightly thinner than I expected, but is very soft and lightweight. Right warmth for the north California winter. It's also very easy for a toddler to put it on himself. Since my son loves it so much, last week I purchased another 3 pairs (1 in 3T, 2 in 4T) when it's on an even deeper sale price.