Babies can begin weaning off the baby bottle by age 6 months which is when you can begin introducing your baby to the sippy cup.
Then by age 1 year old it's recommended to stop giving your baby the baby bottle completely and let them drink from a sippy cup until they are ready to drink from a regular cup.
Some babies continue to drink from the baby bottle until 18 months which is okay but they should not use the bottle past 18 months and if they are still using the bottle by age 2 years old the bottle should be taken away.
A 2 year old should begin drinking from a cup but if the 2 year old won't drink their milk from anything but a bottle then it's better they get their milk even through a baby bottle.
They will eventually give up the bottle but you should take it away from them by age 2 at the latest and get them drinking from a regular cup.