There is an extra hole in the nipple for each stage. Stage 1 nipples have 1 hole, stage 2 has 2 holes, and so on. Baby is able to drink faster as they get older. We switch up a stage when we notice that DS is getting frustrated that the milk is not coming out fast enough. We went to #2 at about 1 month and #3 at about 10 weeks.
Re: Avent bottles when did you switch to #2 nipples?
I am sure it is - it says online they are for 1+ months but doesn't say why to switch. I was curious what other people are doing.