whose age is
The USA started at just below 10%
In 1940, elderly people represented 10% of USA population
which is the highest porportion
which was the highest proportion
There is then a low period of stabilazition
This was succeeded by a period of stabilization
, not until the year 2020 is it predicted to rise dramatically and hit a peak of below 25%,
You could start a new sentence from here to avoid a very long sentence.
, after that, it is anticipated to rocket to a quarter in 2040
Again, this could make a new sentence.