My DS age 6 has this. It is due to a traumatic birth. He was born with a black eye and bruised and swollen head.
This resulted in him not being able to open his eye properly. He has regular eye tests because it is a case of 'if you don't use it, you lose it' with sight and the lid covers his pupil. He wears a funky patch for three hours a day. This helps.
He has a great big smile and fun personality.
He has had two ops, which didn't work very well.