I'm not big on the 'serious cyclist' label, but in the context of your definition, I've been commuting to and from work on a bike pretty much continiously (I've taken some breaks due to circumstances) since I was 13 years old...not counting paper routes before that or riding to school. I'm 47.
__________________
"Let us hope our weapons are never needed --but do not forget what the common people knew when they demanded the Bill of Rights: An armed citizenry is the first defense, the best defense, and the final defense against tyranny. If guns are outlawed, only the government will have guns. Only the police, the secret police, the military, the hired servants of our rulers. Only the government -- and a few outlaws. I intend to be among the outlaws" - Edward Abbey