As far as I am concerned there is a good mix of ages in the North.
Most seem to be couples in their 40/50s but there are lots of other age groups including older.
Certainly not a young peoples holiday with all night bars and discos etc but plenty of evening entertainment if you like conversation with strangers and making new friends
Clothing is what you want, generally for me t-shirts and shorts with sandals and maybe trousers and a shirt on eveningsif going to a "special " restaurant (but not always) absolutely no need to dress up anywhere.
Beaches , especially at Baga can be busy but are not as crowded as the Costas but at Candolim are quiter with plenty of space and there are plenty of very quiet beaches a taxi ride away which cost about £10/12 for a taxi and food for the day.
As far as things to see and do without taxis then I think that I would struggle to find something interesting for a day or two once you have wandered among the shops and bits of markets etc and seen some of local lifestyle.
But as taxis are so cheap that is not much of a problem and you can of course use the local buses, which are "intresting" and very cheap.
Panjim is accesible by bus and free ferry also Mapusa is easy to get to by bus.
The beaches are easily got to by foot unless you pick a hotel out of the way a bit, EG Marinha Dourada where a taxi or free bus is needed to get to beach.
Whichever resort in the North you pick there are always lots of restaurants/shops bars within walkng distance.
I am late 50's and like to relax on holiday but with a bit of something not too strenuous to do occaisonally and Goa fits the bill for me everytime
I cannot comment on the south , not having been there.
Hope this helps you