anyone know the age where our body slowly starts degrading?
i think its like 23 or something.
Then I've already got one foot in the grave.

So we've got that settled, it looks like; although I would like to point out that your body already knows on some level how big a drop you can take, and it tells you through fear. What I like best about parkour is it's an opportunity to explore your relationship to fear. Every fearful encounter while training is a chance for you to explore that and decide if it's real fear from self-preservation, or if it's just an overblown fear in your head that you can train to get over. I wouldn't recommend deliberately climbing tall buildings on a daily basis to test where your fear relationship lies; there are plenty of ways to explore that relationship with other things besides drops. But the point is, on some level you know what you can and can't handle based on your gut fear reaction.
There is no real set height limit that an "average" traceur can handle, because what is an "average" traceur? We are all at such different, individual places in our training that it's hard to determine what an average traceur is. Plus the traceur skill set is so varied that someone who is highly skilled may be stronger at climbing type skills vs. jump/drop type skills. So your question was kind of a non-question, really. I can see your curiosity, but it's not a simple thing to examine, as we can see from where the discussion went.

There are three rules of thumb I have always heard applied to the question of drops, that most traceurs kind of accept by way of general consensus:
1. Train for landings from small drops, and drill them repeatedly; try larger drops once in a great while to get the experience "just in case" but don't make a habit of it (essentially the conclusion we've all been getting at in this thread)
2. Do not climb up onto something that you know you can't safely get down from. Parkour is not just about going from A to B but you must also be able to go B to A.
3. A good maximum height limit for drops, for the first year to three of training, is the traceur's own height. And that's the *maximum.* So like those once in a blue moon drops we were talking about before, should only be from things your height, if you choose to follow this guideline.
Again, they're guidelines, they're not carved in stone. But hopefully it provides the information you were seeking when you posted.
Younger traceurs will probably look at my thoughts on this and think I'm some kind of chicken, gutless, "throws-like-a-girl" traceuse; but for me, I want to still do parkour when I'm 80 and I'm in no hurry to get spectacular at it. I would rather progress very slowly, and be vaulting my walker when I'm in a nursing home, than be Queen of The Videos, vaulting buses at my current age, and sitting in my Barca Lounger, only able to reminisce about it when I'm 80 and in a home. That's just my personal motivation. Plus as has been mentioned, I'm already halfway to my grave anyhow.
