Is it advisable for a 89 years old man to visit Nan'ao Island. I was told it is very beautiful.

If you travel alone, it is not suggested to go considering your age.
If you can travel with other people, you may have a try.
The island is beautiful and is worth seeing.
If time is possible, you can stay there overnight. But if time is not enough, one day is enough.
From ST General Bus Station, there is direct bus to reach that island at around 08:20, 12:00 and 14:00.
I spent about 1000RMB to visit there.