Didache is by no means a Church-wide dogma. It probably reflected the practices of some Jewish Christians at the time when the Apostles still walked the earth and were busy writing Gospels.
Can we assume that simply because Didache does not mention anything about infant baptisms that other communities didn't practice baptisms of entire families, as there is evidence to that effect?
Didache may be an insight but by no means a "Bible." Children were not really much of a subject in those days unless they were children of kings, or were the targets of someone's slaughter, and even then the records if often lacking. In other words, chlidren were not given the important role of decision making, but simply followed what their parents did.
You will also notice in the quote that it says "let him who baptizes and him who is baptized fast, and any others who may be able to do so" but it doesn't say that those who may not be able to do so cannot be baptized! Obviously, children were not expected to fast, as they are not expected now.