A Hot Air Balloon

How many years until everyone
Alive as these lines are typed is gone?
And how many years going backward
Would you need to reach the point at which
No one right now alive was yet born?

The second one is easier, more
Amenable to an estimate—
Even if the soul designated
As oldest at the moment is not,
They’re unlikely to be that far off.

Let’s round up a bit to 120
And predict things won’t change much, as well.
In 1904, no one alive
Now was already born, and no one
Here now will still draw breath

By the time 2144
Is announced, assuming years still count.
Of course there’s nothing about that stretch
On which you could safely bet. What did
1786 know of this?