My daughter was six when she started wearing glasses, and I never had a problem with her keeping them on, because she sees so much better with them on.
Be careful where they are put when they are taken off. Another daughter of mine took hers off when she read, and put them on a low table next to her. The puppy chewed them up. They were under warrenty and replaced for free. She left them on her bedside table once and the puppy climbed up and chewed them again. We had to pay to replace them this time.