Suppose you're someone who's been training hard for 10 years - two years of middle school, four years of high school, and four years of college. After that, you don't run or cross train for two years due to mental burnout, work and family commitments, and/or injury. Before you retired, you were quite good, but not great. Let's say that you ran 60-70 MPW and got a 15:00 in the 5K.
Now suppose that you have an identical twin who has never run before. One day, both of you are watching the Olympics, and you and your twin are inspired to get back in shape. Before either of you start, will there be any physiological differences between the two of you? Also, if both of you do the same training, will you run a sub 17 5K before he does? What if you took 5 years off instead of 2?