Okay so this is gonna blow most modern people's minds, but:

According to David Graeber, in the mediaeval age most adults owned their own means of production. The land belonged to the local lord and you paid taxes on it, but you owned the tools and you had a kind of ownership of the land, cause feudalism is hierarchies of land ownership.

Let's say you're a farmer. As a youth, you'll go and work as a farmhand or a milkmaid on a more experienced farmer's farm. When you become sufficiently experienced, you'll marry. The wife's family gives the husband a bunch of money, and he uses it to buy his own farm. Then he takes on farmhands and milkmaids.

Ever heard of a masterwork? An apprentice artisan, like a blacksmith or a painter, needs to make a masterwork as their final graduation project, and then they become a master, and they're expected to get their own studio/forge.

During the industrial revolution when people started doing wage labour, for other people, as a full career, it had social consequences. People felt like they were perpetually trapped in adolescence. Because you never got to own your own means of production.

So Marx is saying, we used to all get private property as we grow older (well, men used to). But now 90% of us are wage labourers, we're never going to have enough private property to get by. Even if you're, say, a digital artist, you're probably working on a painting app rented from a corporation, you don't own it. You don't own your advertising, you don't own your distribution, you don't own your payment processing, you don't own the house or office you work in.

That didn't used to be the case. 300 years ago, if you were a man artist, you'd work your way up to having your own atelier and do business face to face with your clients. Now they're charging us rent for everything. That's what Marx is saying.