I'm with Katie. If they are able to drink from other methods and are not walking around with a bottle or putting their teeth at risk by sucking all day on it, then no worries. DS had his milk in a bottle 2x a day at your twins age. DS has been drinking from cups and eating with utensils on his own since 1 yr old. He drinks from sippy's, straws, tea mugs, wine glasses, water bottles, fountains, faucets, his hand cupped etc.... !
BUT.... in his words one day, "Could I just have my milk in my bottle today, it is just so cozy..." how can I begrudge that!? There, gave him the bottle of warm milk no less and he was in his "cozy space" for less than 5 min and it looked like bliss, wish I could of joined him!
He goes to nursery school and drinks milk, juice etc...from a cup and never asks for his bottle anywhere, but once in a while when he wants to be cozy at home even though 3 yrs old, go for it sweetie! As a matter of fact, he is the only 3 yr old in his class that knows how to pour the milk into his cup without spilling and some can't drink from the open cup with ease and spill too. The teacher said a lot of the LO's only know how to drink from sippys and are just now working on open cups.
Aisling x