Mine started using a straw cup around 9 months, but she was initially really messy - she figured how to suck, but would draw up too much water and couldn't swallow it all, so it dribbled all over her shirt.
I think what really helped her figure it out was using pouches for applesauce & yogurt. We had to squeeze a little out of the spout, but after a few days of consistently using the pouches, she figured out how to suck from them. After that, sucking from a straw just came naturally.
She finally "mastered" the sucking/swallowing combo around 10.5 months, but she still gets confused and tries to tip the cup up (like a sippy or bottle).
Lexie will only use the straw but it took her about a month also with the fruit pouches to "get it". She doesn't understand how to drink from other sippys, the concept of tilting it somehow doesn't connect between bottles/straw cups. Oh well- less likely to spill!
"I will show you the kind of big sister I will be..."
DD uses straws all the time. If you have a drink with a straw, she'll grab it and drink out of it. I'm not sure when it 'clicked', but she sucks up pouches like a straw too.
Re: Straws
DS: 9/18/12 - 40w5d // DD: 05/17/16 - 40w
"I will show you the kind of big sister I will be..."