How is it possible? We've seen nothing in the show to indicate that the 12 models are 20 years old - to me, that's just a fan supposition. Everything Moore has said about the 12 models leads me to believe they were created at the same time.
Of course, they could have been made 20 years ago or more, but Zarek has been known for probably 30 years. Also, the human cylons act like a younger race. Definitely less than a decade in age.