Is it wrong that I want to date my ex-boyfriend's good friend?

My ex-boyfriend and I are friends and we have this mutual friend that I've liked for awhile, but I haven't made a move because I know guys have this unwritten rule about how their boys should not date their exes. I don't want to come in-between their friendship, but I like this guy A LOT. I actually knew the guy since I was a kid, before he or I knew my ex...so would it be okay for me to finally say something?