We changed at around 2.5 months. She was getting really upset during her feedings and it was taking at least 30 to 45 mins at each feeding. We decided to try it and see if she liked it. Otherwise we were going to stick to Stage 1. Since then she is has been on Stage 2 nipples.
I recently bought a variable flow nipple for my Born Free, which has three settings depending on how you hold the bottle, but is labeled 3+ months. Even the lowest flow is still a little too much for DS, even though he eats like a crazy man. I am glad I bought that one though, because it can grow with him when he is ready. I should add that DS only gets a bottle once or twice a week.
We made the switch just before she was 3 months because it was taking her almost 45 mins. to finish a bottle and sometimes she'd put herself to sleep with it. It took her a few days to get used to the faster flow - she was finishing her bottles in about 10 minutes and seemed to be missing that soothing-sucking time so we gave her a paci right after and she seemed just fine.
Re: Born Free users pls come in.
We switched to stage 2's around 5 months.
ETA: It was taking DD a really long time to eat her bottle and she was getting frustrated, so we decided to give the stage 2's a try and she loved it.
my angel babies: 6/10 (chem. pg), 9/10 @ 10 weeks