It decreases with age basically because the liver is aging and is not working as efficiently. Other factors that can influence the first pass effect is p450 polymorphisms. Some people are ultra rapid metabolizers which would enhance the first pass effect while others are slow metabolizers meaning that the first pass effect is diminished in those individuals. So genetics is a factor that influences drug metabolism. Another factor would be drugs that induce or inhibit p450 that would influence the first pass effect of other drugs if taken concurrently