You know, I've been wondering the same thing with my kids, but I honestly don't think it's a big deal to worry over.
I have a four and a five year old. They both drink out of regular cups just fine, but sometimes when they're playing or watching tv in my bedroom they'll say "can I have something to drink in a sippy cup?" because they want to take it to their rooms, and cups without lids have to stay on the kitchen table in our house. So anyway, lately I've actually been thinking of just giving away all of my sippy cups, that way when they ask I can say "oh, sorry, I don't have sippy cups anymore because you guys are so big that I gave them all away". I wonder sometimes when I give them to them, like thinking they're too big, but it's only a cup, and that's the way I look at it. All of mine have the hard plastic spouts, so it's not like they're chewing on anything soft and ruining their teeth, but I really do think I'm just going to get rid of them. So, my answer to you would be "It's not a big deal". There are many many other issues you could be focused on, and as long as they do know how to drink out of regular cups easily, then I just don't think it's a big deal. But, having said that, I would definitely try to not use them on your five year old as much as you can. I probably let my 5 year old use one maybe a few times a week.....for now.....