Yes, you're right, parents are usually the best judges of such things. Parents of preschoolers tend to be young, though, and subject to family and neighborhood pressure. It's really nice, when you have a feeling that it would be better to wait until the child is 6 to start school, to have the opinions of "expert witnesses" to fall back on for support in the face of subtle and not-so-subtle (pushy grandparents) pressure to conform.

When my oldest was 4, we lived in Brattleboro, VT, which had a screening program for Kindergarten candidates. It was voluntary, in the sense that you had to go out of your way to refuse to do it. Though I wouldn't characterize us as detached or unobservant parents, the screeners caught the fact that our son had astigmatism in both eyes. He had never squinted or sat too close to the TV, or exhibited any other of those typical symptoms of imperfect sight that would have inspired a trip to the eye doctor. He just thought that's the way the things looked. He was delighted with his glasses ("Danger Mouse isn't fuzzy anymore!"). He might have gone another year without it being detected if not for that.

On the other side of the coin, the daughter of a friend here developed staph infections in her ears as an infant. The infections were treated successfully, but the damage to her hearing was permanent. She isn't deaf, but in that neverland called "hard of hearing". Every year was another fight to get her new teacher to recognize that she wasn't inattentive or stupid, but disabled. I tend to believe that, had she lived in a place where hidden conditions like vision and hearing impairments are proactively sought by the school system, her teachers would have been less ignorant of the effects of hearing loss. A school system that goes after such things from the beginning already has a culture that encourages awareness among its employees.

Regards--
Mary Anne