According to an article in New Scientist a couple of weeks ago most of the human body is no older than 15.9 years. The cerebral cortex is as old as you are as is the visual cortex while the cerebellum is slightly younger than you are. Your intercostal muscle is 15.1 years old, your gut (not the lining) is 15.9 years old, your gut epithelium is 5 days old, your skin (outer layer) is 2 weeks old, your red blood cells are 120 days old and your bones are 10 years old.
Now if someone wants to work out a weighting system for those numbers then you could actually work out your average age. Since the cerebral cortex, visual cortex and cerebellum aren't that massive when compared to the rest of the body I would hazard a guess that the average age of the average human body of say a 25 year old person is actually quite low. I'd guesstimate about 10 years. That means that underage sex is a bigger problem than people realise.