He became blind in the middle of his life. So, at mild life the speaker's eyesight failed.
Sonnet XIX is one of 154 sonnets published by the English playwright and poet William Shakespeare in 1609. It is considered by some to be the final sonnet of the initial procreation sequence. The sonnet addresses time directly, as it allows time it's great power to destroy all things in nature, but the poem forbids time to erode the young man's fair appearance.