I think it's just personal choice / preference really. Whilst in a sleep school for my DD at 15mths I got told by one of the nurses that she thought 'kids over the age of 12mths looked ridiculous with a bottle in their mouth'......made me feel great!!! Anyway, my DD was only having 2 bottles a day (cows milk by this stage) at this stage so shortly after took the bottle away and swapped to a sippy cup with no problems. I still see 2yr olds with bottles so I think it's entirely up to you / needs for your baby. I wouldn't feel pressured to do anything, but it was great not having to wash the bottles all the time.