I have a pattern for a baby sweater. It has sizes like 3 months, 6 months, 9 months, etc. Is the 3 month size 0-3 months, the 6 month size 3-6 months, etc.?
Your thought is correct. 
I have a pattern for a baby sweater. It has sizes like 3 months, 6 months, 9 months, etc. Is the 3 month size 0-3 months, the 6 month size 3-6 months, etc.?
Your thought is correct.