Lilah is almost 27 months old, and though she can drink from an open cup, most of her drinks are still in a sippy (she doesn't usually take much liquid from an open cup, and she's a "sip through the day" kind of girl, so to get enough fluids in her, we still give her a sippy). I'm not really sure when or even how we introduced the open cup, though she has always liked to drink from our cups, and the nasty bucket we use in the bath
. I offered open cups of water occasionally from about 20 months or so (I'm guessing). But one day I put an open cup in front of her and she just picked it up and drank from it like a pro.
Sorry, I'm not much help! Just try it out and see if she takes to it. If not, don't push it, and try again later.