At what temperature is it weird to wear a jacket?

Wearing a jacket above 20 C

Whatever you feel comfortable at is fine. For me, I can go out in a T-shirt in 50° weather and feel fine if I'm moving around and being active, but I'm very warm blooded.

70

What's wrong with wearing a jacket when it is warm?

anything above 24 Celsius

Probably once it reaches 70. I'm cold if it's anything below that. XD

60 and below.