As far as I know, thickness and density increase with age and this would cause the plate to be heavier and thus subduct under the other. Does this make sense?
Let's say that two oceanic plates run into each other in a head-on collision. Pretend that one is 2000 miles from its spreading center, and the other one is 4000 miles from its spreading center. My question to you is "which one of these would subduct and why?"
Bill in Miami