After year 5, 100 23 year olds will marry 100 19 year olds. In year 6, when the 23 year olds look at the shidduch pool, they might find a few 24 year olds, 23 yrar olds, and 22 and 21 year olds, but no 20 year olds, because they all got married. In year seven, there won’t be any 20 or 21 year olds; they will have all gotten married in the past two years. Etc.
So after 10 years, your pool of 19-24 year old girls consists of exactly 100 19 year olds.