At what age should you completely transition from bottle to sippy cup?

15 months. The easy window of opportunity seems to be by 15 mos of age, no later than 18 mos. This is in order to protect the teeth from increasing amounts of cavities and orthdonture problems later. Remember we wean the parents not really the babies.
18 months. You should plan to be there no later than 18 months, 15 months is even better in order to protect the teeth.
12-18 months. Ultimately, the sooner after the frist birthday, the better. Longer use of bottles predisposes to cavities, poor eating habits, and ultimately to malnutrition and obesity. Most toddlers resist the transition, but once you finally just throw out the bottle--and they see that you mean business-- they're over it in a flash.
12 months. Baby's motor skills are ready at a year.I recommend hard spout sippy cups as soft spout cups are just bottles in a new form. I tell moms to put water in the bottles(protects teeth) & the good stuff in the cups. I encourage mom to leave several cups around at baby eye level for advertising.Some babies cling to bottles for comfort. Thats ok with me. They can toss it when they are ready to let go.