M.S.
My pediatrician recommended transitioning from a sippy cup to a straw cup completely by 18 months. I did this with my 3 year old and he transitioned easily, really loved the straw cup. My 18 month old has been drinking from sippy and straw cups since before he was a year and I think he prefers the straw. As for switching from bottle to cup, the sooner you do it the easier it will be. I've know some kids who went straight from bottle to straw cup. My absolute favorite is the Foogo stainless steel cup with straw and hinged lid. I also saw that the Safe Sippy (also stainless steel) came out with a straw cup and I absolutely loved their sippy and plan to get the straw cup soon. Hope that helps!