Ben Affleck's co-star Blake Lively made him feel old

Ben Affleck had an eye-opener hanging out with his The Town co-star Blake Lively after she admitted she was clueless about his famed friendship with Matt Damon.

The actor was showing the 25-year-old Gossip Girl star around his hometown of Boston, Massachusetts while making his 2010 heist movie.

Affleck, who turned 40 last month, admits he was shocked when Lively confessed she had no idea the actors grew up together before co-writing and starring in the 1997 drama Good Will Hunting.

He tells Details magazine, "When I was doing The Town, I'd tour the actors around Boston. I was with Blake, and I saw Matt's childhood home. And I said, 'Oh yeah, that's where Matt grew up.' And she said, 'Who?' And I said, 'Matt Damon.' And she said, 'Oh my God! You know Jason Bourne?!'

"She really didn't know. And I thought, 'There it is. The first age of people who are adults who missed the whole Matt-and-Ben propaganda campaign.' Mostly, it just made me feel old."