Term for “extent/proportion of a population [that meet some condition]”
Solution 1:
It's called demographic, a particular sector of a population.
demographic
noun
A particular sector of a population.
‘the drink is popular with a young demographic’
‘All of this places me squarely in the demographic of People Who Tend To Buy Things Online.’
Collins:
2. a section of the population sharing common characteristics, such as age, sex, class, etc
This creaky sitcom must have been conceived by executives anxious to appeal to a particular demographic. Times, Sunday Times (2015)