Adam moved from size 1 (TT CTN) at about 4 months - he started to get easily distracted, fussy, pulling away, seemed to be fed up. I switched to variflow and he was soon downing his whole (6-7oz) bottle in 10 minutes. Have stayed with variflow because he won't take size 2 or 3, for some unknown reason. Only problem with variflow is that the cross in the teat rips open further after a while and the milk pours out - I've had to replace my set of 4 about 4 times now.