I always microwaved my bottles I just made sure I shook them so there were no hot spots. But my kids were off formula at 9-10 months ..... As soon as they ate all four food groups and a substantial amount and I switched to homo and put it in a sippy cup .... I never allowed my kids to have a bottle after 1 year. I had one baby start at 9.5 months and I made the formula here with a supplied bottle which I washed but I switched her over to a sippy cup by 1 year and to be honest I think they still give get a bottle at home but she hasn't had one here in 4 months. Next time I am faced with formula and bottles I will have the parent send them full and I will just return them at night