I wouldn't worry about it children do things at different times. My daughter loved her bottles at that age even though she was fully weaned, she could easily drink out of cups but if she saw milk was in it she refused it. I didn't really see the point in distressing her by forcing her to do something she didn't want to do when the bottles weren't causing her any harm. She decided when she didn't want them anymore and it happened over night when she was about 20 months.