There is a general misunderstanding with a current population trend. The only reason why it is growing ( for now) is because less people is dying, but also less people are getting born, and this is not an accident.
Maybe the average is still in favor of birth rate, but that's because of developing countries.
I know some will say, there is too many of us, we are destroying nature... blah, blah, blah... but number of people itself doesn't have anything with that. It is how the technology is used that is making whole difference.
Also, we don't have enough food, energy, water to sustain so many people... blah, blah, blah... yeah, but somehow the biosphere and nature have all that much to sustain all the cows, chickens, pigs and numerous other lifeforms that by numbers are far greater than us? That's weird...
Put aside a philosophy, the propagators and supporters of that theory that we will live happier and healthier life if our numbers decline, are really not ready for a big surprise.
It is already happening and the people who are young adults or adults today, will be a batch who will feel the economical consequences of that change.
I live in the country that has an aging population. What does it mean? Well, it means there is not much young people out there and things slowly going to hell. The only thing you need to do is go into the traffic and you will see what I mean.
Now, put it on the next level. Imagine that both pedestrians and drivers are 104, and you are the sole young person on the road with a freaking bicycle. That's how an aging population feels like.