My girl started drinking from straw at 6 mths. What I did was just put it in her mouth and make sucking spounds. For the first ten minutes she just bit the straw and play with it inside her mouth. But after that she started sucking but was surprised by the water and she didn't know she gotta swallow. After trying afew times, she finally sucked and swallowed.
I read somewhere babies as young as4 mths can drink from straw. I didn't believe till I saw my friend's baby drink from straw. She is 4 months now.