I had dss (ds1 5, ds2 3) tested as dh and I are both short-sighted. Turned put ds1 needs glasses and optician said I came at a very good time as there was a still a chance of completely correcting his vision.
kid I am very surprised they said that. Ds2 was three when he was tested, the optician showed him pictures to name - luckily his eyesight is good. And for children who are difficult to test they put in eyedrops and then examine the eye itself to determine eyesight problems.
I was very aware of the need to test as children often don't realise they can't see properly. I remember squinting at the hymnsheets in assembly when I was at school and it never occurred to me that I needed glasses. I was just surprised that everyone else could see them clearly.