Main reason to change from bottles to sippy cups etc after certain age is....

[megcleary]

teeth? Is that right.

tiny bump

Yes - milk (or worse, juice) stays on the teeth more when taken from a bottle than from a sippy cup.

I thought it was to help them learn the sipping mechanism, I.e. To break the association that to drink means to suck. Therefore helps to get them onto an open cup more easily.

And also because certain drinks like fruit juice can erode teeth, and having in a bottle means they are more likely to be in contact for longer