You don't need to look too far for examples. Even on this very site, there are quite a few people who want to make a huge difference with their lives in one way or another before they're 30 or 40. Because they know they probably won't have another shot at it later on. What would that number be if they had an infinite number? Many wouldn't probably ever get around to try.
How many people try to make a difference in the world, fail (because it is hard) and then capitulate into working a salaried job managing database queries for MegaCorp...because it is safe and they have a family to feed.
If you knew you would live until 300 and remained reasonably "young/healthy" during that time you probably wouldn't start a family as soon.
Right at the higher levels of human achievement, I believe that it is curiosity and inquisition that drive us. Certainly impending death can focus people, but I do not believe it delivers the spark that lights ingenuity.
“Remembering that I’ll be dead soon is the most important tool I’ve ever encountered to help me make the big choices in life,” - Steve Jobs
A few months ago, this was one of the biggest quotes around the internet. For a lot of people, death is a catalyst to do something that matters. In the biography, Jobs talks a lot about how he's in a hurry because he feels like he's going to die young.
No it doesn't. The death of young children, or victims of accidents or violent crimes for instance, doesn't force them to chose anything. The notion that you have limited time, might make you want to prioritize. You can hardly speak for anyone else, or what makes them choose anything.
I have a limited time on earth, yet I chose to spend it with you. Cuddling on this couch. Watching a movie. I think by the finite nature of our existence it imparts a little bit of significance to any sacrifice we might make to our own utilitarian maximizing.
Well, you didn't convince me there. We humans are lazy by nature. And you are spending time with him/her cuddling on your couch because your life is not in immanent danger. In other words, you are not assuming you will die tonight, but rather that you will go sleep, like any other night, and wake up next day (although one day you will be mistaken, but then it won't matter anymore).
Have there been an earthquake or your place would be ate alive by fire, you would be running away like crazy and the last thing on your mind would be to cuddle on your couch.
Further there is not much you can do to postpone the judgment. Sure you can break up with your loved one and lock yourself in a library for 10 years, grasp some wisdom, perhaps invest money or built a company, sell it ten years later for half a billion $ and invest all your money strictly into life-extension projects, but by now most likely you know the probability of achieving all this is so small, that you would rather cuddle on your couch just for a little bit longer.
Why do you think limited time give us meanings?