I live on the Kent coast and in my small town it is very rare for people not to look after their garden and make the most of them.
Most of the gardens I see have a lot of plants/flowers/trees in them as well as things like bird baths, bird feeders, little statues, solar lamps and of course garden furniture. The only empty garden I have ever seen is in a new built estate when I was house hunting....
I really don't think it has anything to do with age/being retired either.
I see lots of families with young kids who spend a lot of time creating a nice outdoor environment.
As was raised in the countryside so for me it is bonkers that people would just have empty 'gardens'...
Mine as vegetables in raised beds, herbs, fruit trees, flowers, compost bin, garden furniture, shed, solar lights, bird feeder, bird bath.
I am not anywhere near retirement by the way.
I enjoy gardening: it is a good physical exercise, good for my mental health, it makes sense to grow my own products and it is a joy to spend time outdoors with my pets when the weather is warm(ish).
It really doesn't have to be that expensive or time consuming either.
I got all my garden tools free from Freegle for example and you can buy seeds and plants in supermarkets these days.