https://content.tfl.gov.uk/60-plus-london-free-travel-national-rail-map.pdf
Doesn’t distinguish the Stansted Express from any other NR service so I would say yes. I tried to buy a ticket on the line to Cambridge North from the boundary of zone 6, but found it much cheaper to book it from Enfield Lock (last valid station) instead. I don’t know why this should be.