Heroes costars Hayden Panettiere and Milo Ventimiglia have called it quits, according to Usmagazine.com.

"It was a lifestyle conflict," a source close to the couple tells Us, adding that the split happened within the last week. "They were in very different places." The source added, "They are still on friendly terms. There are no hard feelings."

The word is Panettiere, 19, "is young. She likes to go out in the Hollywood scene and that's not his style."

The two, who began dating in 2008, had been an open secret for months - in part because of their 12-year age gap.