What closing do you use at 15C? After a few years in Norway I can be OK in a t-shirt at 18C. Anything below that still requires a jacket or sweater to stay comfortable.
Not OP, but here in Austria I wear t-shirts for most of the year. I even used to go out in just a shirt on warmer winter days with only light snowfall, but nowadays I really notice the cold when I do that. Everything above 10°C is very comfortable to me.
Among my friends and relatives I am the exception of course. Most of them dress in more layers, some even in summer.