Lets say there a thousand humans (a really small earth).
If we distributed them roughly across ages so they balance out at 100. I could have a:
- very dense demographic with lots of 100 year olds.
- a split demographic with many young individuals say 10-20, and many older individuals 180-190.
- or some kind of flat demographic ~5 people of each age between 0 and 200.
That is the distribution you've described. Take your pick.
If you want something more realistic... (and grossly oversimplified).
Then lets consider mortality. Without aging doing any killing by itself, you'll find that we die purely from disease and accidents. This probably won't change the mortality rate very much. I'll draw some figures from Australian adult deaths. So roughly 0.0468% to 0.0782% of the population dies due to something.
That's about 46-78 dead humans a year (in our thousand human population). So about that many births per year.
- 78 one year olds
- 73 two year olds
- 65 three year olds
- 63 four year olds
- 20 five year olds
- 15 six year olds
- 80 six year olds
- 45 seven year olds
- 46 eight year olds
- 48 nine year olds
- 20 ten year olds
- 72 eleven year olds
- 12 twelve year olds
- 22 thirteen year olds
- 46 fourteen year olds
- 23 fifteen year olds
- 31 sixteen year olds
- 87 seventeen year olds
- 44 eighteen year olds
- 38 nineteen year olds
- 22 twenty year olds
- 56 twenty-one year olds
- 64 twenty-two year olds
and because of the small population cap, 51 people are executed at random. But if you raised it, to something more manageable, you'll see that this roughly makes a flat population that will slowly taper out (at the rate of a 5 hundredths of a percent).
Lets see what this population looks like, but with no ritualised forced deaths, and every member living till natural causes (like accidents) kill them. Lets also keep that median straight on top of 100 (roughly speaking).
This is represents two triangles and a rectangle like:
* * *
A* * *
* * *
* * *
So assuming D = 0.0005
So the oldest thing is roughly 341.42 years old.
And their lives one 17th of a newborn. For a median age of 100. (Statistically speaking). That is a total average population of 341.42 * 0.17071 / 2 ~= 29.141 individuals.
29 people isn't a satisfying population. Its an almost extinct one. Lets take the approximately 7 billion people alive today, and give them a shot of immortality. and institute the replacement only birth rate, with the constant death rate of 0.0005. (Lets say immortality makes people more adventurous and take on risks, without learning much about avoiding them). Fast forward many years and allow the natural randomness to even out a little, (so that I can use the simple triangle model and ignore every other event).
* * C
- D = 7,000,000,000
- D = A * B / 2
- C = A / B
C = 0.0005
A/B = 0.0005
- A = 0.0005 * B
- 7,000,000,000 = A * B / 2
- 7,000,000,000 = 0.0005 * B * B / 2
- 7,000,000,000 * 2 / 0.0005 = B * B
- B * B = 28,000,000,000,000
- B ~= 5,291,502.62
- A = 0.0005 * B
- A = 2645.75
So natural causes gives the oldest person at about 5.3 million years old. Most people would have "accidentally" died by now.
Also there are around 2645 new infants born every year. This means a mere 6 average-large sized schools of each level of education are sufficient for educating the entire population. Imagine that, overnight most of the worlds schools can just be shutdown. Of course there will now be a large upswing in golf-course requirements.
Hope this helps paint a very crude picture.