The Prince of Persia: The Sands of Time star, who is rumoured to be dating 20-year-old country singer Taylor Swift, will mark the milestone in December (19Dec10) and he admits he's eager to become a father after spending so much time with his sister Maggie's daughter Ramona.
He tells USA Weekend, "It's incredible to see the traits she has. She mimics things that my sister or my brother-in-law do, and it makes me laugh. I just adore her.
"Children are really where it's at. That's what I feel turning 30, more than I ever did."
Ramona is actress Maggie Gyllenhaal's four-year-old tot with husband Peter Sarsgaard.