I took another approch to extimate my market size

Increasing population countries:

system equations : x+y =300 mil ; x= y+0.5 with x = 4 classes of 1.5 and y= 4 classes od 1. It comes up to 45 mil in each of the classes ( 0-9...) and 30 mil in each of the classes ( 40-49...) 15 years are requited: 1-2,2-3,3-4,4-5,5-6,6-7,7-9,8-9,9-10,10-11,11-12,12-13,13-14,14-15,15-16,16-17 (it is not 17but 16+364 days).

5milion per age from 0to 39 comes up to 5x15(age classes) = 75mil

In declining population: 5 classes with 1y base = 15 mil in each class

15 classes x 1.6 mil in every age group = 25 mil.

It is more intuitive like this in my opinion.