The beloved rom-com is coming up on its 15th anniversary.
The 51-year-old actor took a walk down memory lane when ET's Kevin Frazier asked what he could recall about working on the movie with Garner. In the film, Jenna Rink makes a wish on her 13th birthday and wakes up the next day as a 30-year-old woman (Garner).
"We had the sweetest time," remembers Ruffalo, who played Jenna's best friend and later boyfriend, Matt Flamhaff. "[Jennifer] was a big star then but I was kind of just starting out. She was just so lovely."
Ruffalo says everyone was on board with the movie's message of accepting who you are and not trying to be someone else.
"We were making this really sweet movie that had a great message for young girls," he gushed. "We were all sort of coming into our own in that moment. We really loved that story, and Jen was just transformed into a 13-year-old girl. It was really amazing."
Fifteen years later, Ruffalo is a big star currently playing The Hulk in Avengers: Endgame, hitting theaters on April 26. When asked whether he could see Garner in the Marvel universe, he admits, "I'm surprised she's not really here yet, actually. Love Jen Garner, by the way."
In addition to 13 Going on 30's big anniversary coming up on April 23, another cult classic film, Troop Beverly Hills, recently celebrated its 30th anniversary with ET. Take a look: