T.D.
Both my kids could use a straw by 4-5 months. I believe this is because they are breastfed. From what I've read the sucking skill they use to breastfeed is a lot like the sucking skill a person needs to use a straw so breastfed babies will often be able to use a straw sooner then a sippy because it's just a lot more of familiar sensation for them.