When someone asks "How are you" or "How have you been (dong)? we can respond in a number of ways. We never respond "I'm very fine." You can respond by saying "I'm (really)good, great, fantastic, wonderful." "I'm doing good, great." "I've been well. " "I've been doing good,great. " I'm cool is more informal and is used with people we know well, like our friends.

I am very well, thank you. I'm cool is slang and can be used with your friends. I am in my 50s and I say ' I'm cool with that ' all the time ! :-)