Either is correct, depending on the context. It all depends what you mean by 'generation'. If you're talking about genealogy, you are of a different generation. This is because you are of a different generation within your family. If you're talking about society as a whole, you are of the same generation in that you both belong to the group of people born between certain years. Sorry, but this is a deficiency of the English language. We use the same word for two different concepts.

That's not a generation difference, as a generation is "all of the people born and living at about the same time". So, you're the same age and the same generation. So, what you can say if someone asked: "is that your brother", you'd say: "no, he's my uncle, we're the same age though", and people will understand the meaning you're referring to. Hope this is helpful.