I was trying to make a list of all auras the Z fighters had in Dragon Ball, and knowing which episode characteres started to have auras will save me a lot of time. Which episode of Dragon Ball / Dragon Ball Z characteres started to have auras?
It really depends on how strict of a definition for aura you're going for. What I mean is it would be one of two events:
First, it could be when Master Roshi first performs the kamehameha in the original Dragon Ball series. He has a slight white aura around him. This is the first example of ki in the series so it may be what you mean. See: YouTube video example
Second, a more conventional aura would be when Gohan headbutts Raditz in the beginning of Dragon Ball Z. When he breaks out of the attack ball he gains a read aura and then proceeds to headbutt Raditz. See: YouTube video example