Was Snape older than Lily?
Snape was the same age as Lily, making him 31 at the start of the series and 38 when he passed away.
How old was Snape when Harry start Hogwarts?
And we know thanks to Professor Trelawney that Snape overhears the prophecy that Harry would be Voldemort’s mortal enemy, meaning he first interviewed the year Harry was born. Presumably he started that fall, so Snape was either 20 or 21 when he began at Hogwarts.