Age 3. Opticians have tests with pictures so the child doesn't need to be able to read.
It's not just about whether they're getting on ok: Visual processing pathways are developing in the brain during early childhood, and if there's a problem with their eyes (or often only 1 eye) then they don't develop the neural pathways correctly. The result is that even with corrective glasses they still can't see. This is called amblyopia, and it affects about 3% of the population.
It can be improved if you catch it in time by wearing glasses and then patching. But by far the best results in correcting it are if it's discovered by the age of 3. Up to about age 7, they can usually get a good improvement but after that, it's much harder and slower (although even adults can get some improvement).
It's surprising the number of adults who have very limited sight in one eye - they just don't mention it. You only discover this when your child goes out and about with a patch!
You can get by fine most of the time with only one eye, but it has implications for speed and ease of reading, and also ability to see 3d. And most importantly, it means that losing the good eye would have very serious consequences. Amblyopia almost doubles the person's risk of complete blindness at some point in their life (up to 1.2%)
It's so easy to get childrens eyes tested, and it could save your child's sight. It's even free. I don't understand why this isn't better known and pushed by HVs etc.