whenever they can sit up and hold the cup by themselves, 6-9 months. Just buy a bunch of differnt brands, every baby is different. Try each one til you find the one that you like. You can go somewhere like walmart where they are all cheap and just get a bunch. I think I went through 6 styles before I found the one that we like.
i started w/ the soft spout that Nuby makes (walmart)...he was about 9 months old i think...he didn't like it at first and it only had water in it...but he got used to it so that by the time 11 months came all his formula was in a cup not a bottle.
My DD wouldnt take a bottle. We started her on it at 3 mos. w/ H2O and she loves it. We got it from Walmart it is for 3+ mos. I BF my DD and she would take a bottle from the sitter while I was at work.