“Marx was just one of a number of people trying to make sense out of the extraordinarily horrendous economic and social conditions of the masses in the 19th century and find a way out of them.
Side note: this is also where the prevailing myth comes from that children today are starting puberty earlier and earlier.
At the same time as Marx is observing the conditions the masses are living in, doctors are starting to keep records when puberty starts and finishes, and when girls get their first period.
Only conditions are so inimical to normal childhood development, malnourishment, pollution and disease ensure that puberty starts later than at any time before. Of all the skeletons archaeologists dug up in London from that time, around a quarter of all those who died in their mid twenties had not yet finished puberty.
Emile Zola's novels describing the conditions the poor live in in France at the same time, have various young characters whose delayed sexual maturity is remarked upon. There is also IIRC quite a lot of sex with no worrying about pregnancy among the youth because the girls and young women in their late teens are not yet menstruating.
It's difficult to truly understand how awful the conditions are for the masses, but if you consider that archaeological evidence shows that two thousand years ago children started puberty between the ages of ten and twelve, and a thousand years ago they started puberty between the ages of ten and twelve and five hundred years ago the same, then the industrial revolution changes everything. Suddenly children start puberty much much later. The average age at menarche today is 13 years. When Marx wrote his analysis of capitalism, the average age at menarche was 17 or 18.
(Of course, life expectancy also takes a hit at this time.)
Anyway, in the 170 years since then, conditions have steadily improved. By the 1950s the average age at the onset of puberty had pretty much returned to the human norm of ten to twelve, and the average age at menarche was around 13 again.
There has been very little change in terms of these milestones in developed countries in the last 60 years or so, although where living conditions remain poor, puberty and menarche still start later. Rapid improvements in living conditions during the 20th century went hand in hand with strong downward trends observed in the onset of those milestones of adolescent development.
Understandably, researchers who collected the data in all this time grew very alarmed at the ever more rapid onset of puberty and menarche, thinking that the data collected during Marx's time was the norm and everything else since a deviation.
In actual fact this is a basic data collection error.
Illustrative though of why Marx would have been so concerned about the conditions the masses were living in. Because they were so harmful to life, some fundamental milestones of child development were badly affected.