The denim jacket is a little bit younger that jeans: the first jeans was made at the beginning of the XX century in the US by Levi’s. Like jeans, denim jackets were intended for cowboys, miners and railway workers as a kind of protective clothing.
It can be worn with a simple T-shirt in summer and over a sweater in autumn. In winter the denim jacket can be put under parkas or even a coat. Moreover, some men manage to put on a denim even under a blazer, and surprisingly, it looks good.
A set of jeans jacket with jeans is usually called a “Canadian tuxedo”. The main thing is that the jacket’s color should be markedly different from the jeans’. But do not wear more than two denim items at one time.
Being originally a working clothes, denim jacket can be oversized but now it looks worse than ever. At least, everybody is going to decide that you like the 80’s. So try to avoid it.