No particular reason or specific milestone. I just decided to give Jace some milk this month. He turned 11 months old and was showing signs of not caring to nurse one way or the other this week. My supply started to drop so I thought I should try out something else in case he is deciding to transition. He didn't guzzle like he typically does, but he WAS all about that sippy cup. He quite enjoyed drinking out of it. I laugh because Rylen would have nothing to do with holding his own bottle much less drinking out of a sippy cup at this age. So different are my boys and so fun to learn about them each day!