Around 6 months, DS2 started taking forever to drink his bottle. He has bad reflux, so we eased him into the faster Level 3 nipple. I started with 1 nipple a day and then only during the day and then all bottles. You probably don't need to do that, we just did because I was trying to be careful with his reflux. Anyway, it was a little after 6 months and he is totally on them now at 7 months. They can be ready sooner or later than that though.