My son was born with a squint and has been having orthoptic examinations from a very early age – glasses since he was about 2, patching and then surgery. And it wasn’t that big of a squint to begin with.
My layman’s understanding is that there is a window of opportunity when vision is developing to get the lazy eye to pull it’s weight, so to speak. Once you’re 7,8 then the eye is developed. If you don’t do anything to treat the lazy eye then it will remain under-utilised and vision will deteriorate on that side long term. 3D vision will also be poor.
You can have squint surgery any age, as that was explained to us as chiefly a cosmetic operation. It corrects the turn of the eye in line with the glasses you wear. So our lad no longer has a squint when wearing his glasses, due to the surgery, but will always have one when not wearing glasses or contacts.