At what age should a baby be able to tip their bottle?

what is the average age that an infant should be able to tip their bottle by themselves? for numerous reasons my daughter is 16 months and still on the bottle (whole milk, not formula), but she isn't able to tip the bottle herself. So, I have to hold her in a reclining position (almost laying down) or hold the bottle myself and feed her. I am having so many issues trying to wean her with a sippy cup, I have been trying for months now and I have no idea what to do. Do you think it's normal to not be able to tip the bottle herself or tip a sippy cup? she can't seem to understand or know how to despite showing her many many times.