Aaand I’m wrong in at least one respect in that a TfL zonal season ticket does come at reduced price for 16+ Oyster card holders.
A station to station season ticket with “16-17 Saver” railcard is still cheaper.
16+ Oyster zone 2/3 annual season = £576
16-17 Saver Clapham-Wimbledon season = £390
As I say, it would be cheaper still if you only bought tickets covering term times.