I started mixing in whole milk in her formula just after she turned 10 months old... I’ve heard of moms starting as early as 9 months old but I felt maybe that was too young... For me, it wasn’t a lot of milk though. Only like 1-2 oz per bottle. Just so she could get used to the taste. Now she’s 11 months almost a year old, and I mix the formula in a bottle of whole milk every so often and she loves it. I’m pretty much positive it won’t be a hassle taking formula away from her once she gets to a year old. (Btw, she didn’t respond negatively to the milk, I didn’t notice a difference in health/mood etc when I introduced the whole milk).
12 months but you can start to give her little sips now. My youngest was a little stubborn he had formula till 13 months but he also drank whole milk just took him time to get used to it.