I've got an app on my computer called "
Memento Mori," which is Latin for "Remember that you will die." It's only $1 and I recommend it. You plug in your birthday and an age at which you expect to be dead and/or have to make a hard decision or something. It just sits up in your menu bar constantly counting down by the second and telling you exactly how much time you have so you can't avoid being mindful of it.
https://itunes.apple.com/us/app/memento-...1393834268
I've set mine to 35 because that's when I've decided I need to make a solid yes or no decision on whether or not I'm going to have a wife and/or children in this lifetime and act immediately on it if I haven't already. In the meantime, I know exactly how much time I've got to square away my career stuff, check unmarried childless guy things off the list, and figure out how exactly I want to raise kids and lead a family if I do, such as religion, values, location, schools, maybe
jus soli if my wife and I are from different countries and want our kids to have a 3rd passport (or birth each of our kids in different
jus soli countries so they can all buy property and own businesses around the world in each other's names when they grow up), etc.
It's not because I'm trying to save the species or anything (the future of humanity does not hinge solely upon my actions) and a low national birthrate in one era doesn't necessarily mean that all future eras will have low birth rates (and, even if every Japanese couple only has 1 kid for the next century, there will still be more Japanese people on Earth and in Japan than there ever were for most of history). The planet also has a biological carrying capacity which is estimated at around 11 billion from what I've heard so I don't necessarily think that every single guy must or is destined to spawn as many children as possible solely for the sake of perpetuating the species.
I just figure that's around the age at which I'll still likely be able to raise children properly and not be in my 70s by the time they are graduating from high school (assuming I even live that long). I'm not so much accounting for grandchildren in that because I'm not taking it as a given that it would happen even if I did have kids but that's something worth considering too as we will each only survive for so long and most of us probably won't see 90. In fact, statistically, most of us likely won't see 80.
Who knows, maybe I'll convert to Islam and move to a country where polygamy is legal and accepted so I can produce as many offspring as possible in the shortest time because I can have more than one woman pregnant at a time without being an absent father paying child support and juggling several families who all hate him. Maybe start my own dynasty and put my last name on stuff throughout the city and province. Could start a famous noble family who people refer to by their last name and origin city. "Do you know the Bamako ____s? They own everything around here." "The ___ name is all over the Moluccas."
In seriousness though, men don't have a fertility clock in the same way women do but, like women, we do have a certain number of years in which we could see children grow up, still play an active role in their upbringing, and possibly be around to see grandchildren. Momento mori.