Tarjeta Dorada: Good for buses in Spain, too?

Generally, I believe not (there may be an exception on the handful of "intermodal" journeys, but I doubt many on here take those).

However, you do get a discount on commuter ("cercanías") railway services within cities.

In past it has only been good on trains but that was at least five years ago. But the buses are pretty cheap.

Buses are run by many different private operators, who will each have their own policies and likely have no impetus to tag along on the railway product. Senior rates on www.alsa.com, for instance, are based purely on being age 60 or older. You could be asked to show your passport as proof at boarding time, if they don't believe you.