How old was your babies when they started using sippy cups? My sons nurse was upset that he wasn't using one at his 4 months appointment and basically said at 4 months hey should be able to hold and drink from the sippy. He's rolling (he can do it but just doesn't care for it usually), trying to crawl, trying to sit up, he can stand with assistance so I don't think he's behind in what he should be mastering