I'd think his forties would be a good guess as well. I know in the United States, you have to be at least in your forties to run for president, so I don't think the Minister of Magic could be too young.
As for not being in the Order the first time, it was, after, a secret organization. It could have been that he didn't even know about it. It does seem like a bit of a Gryffindors club. If he wasn't in Gryffindor and didn't have many friends that were, it becomes even more possible. I always did see Kingsley as being a Ravenclaw in my mind. Or maybe he didn't even go to Hogwarts. Even less connections to the organization.