what is the lowest temperature you would walk in with a t-shirt, what is your threshold?

today i went outside in 12°C (53.6 Fahrenheit), wearing a t-shirt & vest, and it didn't bother me much, some would say that is cold, and I noticed that most people were wearing jackets/coats. I think any lower than 12°C would make me put on a jacket.

How about you?