Well DD is tiny, very tiny. I just weighed her 25 lbs. She is about 35 inches tall too.
She can still fit in 18 months clothes, but I am not buying anymore.
24 months or 2t for her for now on. The funny thing is she fit in a lot of size 2t dresses and shorts last summer, and will again this summer. We had to go back in sizes for the winter (my older DD did at that age too.) We can even do 3t in dresses and shirts (they can be big no problem.)
She is 3.5 now. Will be 4 in Aug.
She can still fit in 18 months clothes, but I am not buying anymore.

She is 3.5 now. Will be 4 in Aug.