In response to this, I'd argue that Marxism stipulates temporal dynamics which have direct and necessary implications for both historical cohort and age. A materialist conception of history, for example, will describe various stages of economic development that a given society passes through - and these stages will be generally sequential, mapping roughly onto a succession of historical cohorts. The particulars of these cohorts will of course be historically contingent, but as with classes, they are defined by specific relationships with the means of production.
Trivially, for instance, we could talk about a "feudal generation", defined as the cohort born in a society dominated by a feudal economy, and oppose that to a "capitalist generation", similarly defined. Such generations, so defined in relation to certain regimes of production, are just as real as any class - in fact, they are mostly just a different way of talking about groups of classes, with "capitalist generation" conveying the same information as "the proletariat and the bourgeoisie".
The advantage with this way of talking about Marxism is that historical cohorts allows us to contemplate age dynamics associated with the material progress of history. For instance, consider this famous passage from Marx's Manifesto:
The bourgeoisie cannot exist without constantly revolutionising the instruments of production, and thereby the relations of production, and with them the whole relations of society...All fixed, fast-frozen relations, with their train of ancient and venerable prejudices and opinions, are swept away...Note that the relations of society that capitalism sweeps away here are not merely feudal - they are definitively ancient. They are necessarily associated with age cohorts preceding the capitalist generation. Thus the sequentiality of various stages of economic history necessarily implies conflicts that will be generationally inflected. This point will presumably be most relevant during stages of revolutionary transition; more significantly, it establishes the general (and I think obvious, though evidently controversial) point that class struggle, proceeding forwards through time, will have historical dimensions that end up mapping onto age.
Such temporal dynamics emerge not just over the broad course of history, but within the quotidian operation of capitalism as well. Indeed, Steve Keen argues that this temporal conception of capitalism was one of Marx's great insights: capitalists, he notes,
in general ignore processes which take time to occur, and instead assume that everything occurs in equilibrium...[but] the process Marx describes was based on an accurate view of the overall structure of the economy...Once we attend to the fact that capital accumulation, for example, is a process that takes place over time, it's easy to see how the old have economic advantages over the young sewn into the very fabric of the material economy. Are these advantages decisive? Of course not. Are they irrelevant to a discussion of the various factors that contribute to power and oppression? Nope. As with any such analysis, ageism needs to be understood with all of the usual considerations of proportion and priority at hand, but the role of such dynamics in Marxist theory cannot simply be dismissed or ignored.