With group care, I don't really switch to open cups anymore. They go from bottle/sippy cup to a cup with a straw or a water bottle.

If there's only a couple one day or we are at a restaurant than they get an open cup but other than that, it's not really an option here. If we have our tea party, it's an open glass teacup and they all do fine with it but usually one gets spilled lol

As far as when they're ready though? It varies with each child and how clumsy they are at the table! haha