Running ultra-marathons is what comes to my mind also, because this is a rare kind of activities where age is at your side. No 14-years-old can run 100km a day without requiring a medical help, because of kinda another structure of muscles.<p>To tell the truth, if you have not picked some skill which you are going to be a world level of, in your 10s, then there is no hope you will do it in 30s. Your question is awesome to see how little of people (even such brilliant ones as HN crowd) can see this obvious thing.<p>Reading comments is so funny, everybody has their confirmation bias. For example, from my PoV the easiest stuff you can do is metal cutting/wood turning. Just buy a lathe, solve that safety issues for your eyes and fingers (this is the hardest part, no jokes), read few tens of books on metals, than work few years with a more and more competitive master - volia, you are a world-class metal cutter... Then I remembered, that not every child use to have the skills I have.<p>If you have not hammered a few thousand nails in your childhood than you need to hammer tens of thousand in adult age just to achieve my level of hammering. Obviously to hammer a thousand nails in childhood is in extreme way simpler than to hammer ten thousand nails in 30s. Another example of sad trend of being older is just a reading books. If you don't use to read books in your childhood, there is no way you will start it in your 30s, having such issues as: bad eyes, hard work, big house to take care of, demanding spouse, growing kids, and of course - lost ability to enjoy reading.<p>OK, while writing this comment, I realized what is the best answer to your question. This is <i>aging</i>. Any 30-yo (who has not done too danger things for longevity like drinking too much of alcohol or having a contact with too danger chemicals) can pretend to compete at a world level about who will have a longer life. I am really convinced that a significant amount of today's 30-years-olds from HN crowd will live to 100 yo, which is a world-level competition from all points of view.