This line is not from Paradise Lost, but from his sonnet known as "On His Blindness." Milton was born in 1608. He penned "On His Blindness" around 1655. (He was completely blind by 1660.) The phrase "'ere half my days" means "before half my days [are past]," which would mean that he had "spent" his "light"--his ability to see--before he was half as old as he was when he wrote the poem. He was, at the oldest, 47 when he wrote the sonnet, meaning that he'd lost most of his sight by the time he was 23. That would place the time of his blindness onset on or around 1631. Since then, at least visually, his world has been "dark and wide."