I read the Madrid Metro website and none of the fare tables I can find seem to mention age. Most other major metro / underground systems that I've seen charge less for children and/or seniors.
Does Madrid not do this? Why not?
About the only difference is noted at the bottom of the page of ticket prices:
Children under the age of 4 may travel without a ticket.
So yes, some children get a discount, but not all.
However, there is a tourist season travel ticket, for x number of days which you can purchase, for certain zones and various combos. For this, children under 11 get a 50% discount, as indicated on the page.