As long as you wake up in the morning with something to look forward to, something you are going to feel good about doing in the future, you will be able to get through anything. The whole thing about 'wasting the crucial years of your life' applied to a time which has now passed. The Y Combinator observation that it's getting cheaper every day to start something potentially world-changing by making something a lot of people really want is not going to go away, just because you're older. Yes, your energy and health may decline in the coming years, but that could happen even if you were working on the startup of your dreams right now. Never imagine that you're going to miss the boat: when you're feeling ready to do another startup, that will be the right time for you. This notion of whether you have a 'window' in your 'lifespan years' as far as starting startups is concerned might have once been 100% true, but if anyone tells you it's still true now, you cannot use expect the past to be a reliable guide.