Yes, I think they'd be very good (without experiencing them myself) based on what I've seen in the Subi newsletters. There is excellent public transport plus so many shops/cafes/restaurants that you'd have a good quality of life without needing to drive – just walk everywhere or catch the bus or train. There are several hospitals and multiple choices for doctors/dentists too. Activity wise there are regular movie nights including lunch (around $10 I think) at the Community Centre aimed at seniors which provide social opportunities, as well as senior fitness classes catering to all ages/abilities (everything from more active classes developing flexibility at Lords Rec Centre, to chair-based exercise classes suitable for those with impaired mobility; classes are aimed at increasing balance, coordination and fitness). Other than the regular seniors-specific activities organised at the Community Centre aged people would probably enjoy the free summer sunday performances (music, etc) and the local parks and markets are nice.
I think the walkability is the biggest benefit – not being able to drive due to getting older and feeling less confident on the road could otherwise limit the amount of interaction an aged person has with other people and their community. In Subi that's not an issue at all.