You were the younger of the two and probably looked up to him. You are probably more advanced socially, he less advanced -- so in terms of social ability, you might have been on par and you still might have even been providing him with someone he could not achieve on his own. (You'll see this often also in terms of who male INTPs marry: partners who can carry the more social aspects of adulthood not just within the marriage but when interfacing as a couple with the rest of the world.)
You were also probably pretty positive, an avid learner, asked lots of questions. One of the typical "roles" that the INTP falls into is sage. He likes to impart insights and information, it's a natural character traits; and I'd assume you were just soaking it up. So the dynamic worked for both of you.
INTPs don't really seem to care much about age. It's whether your thinking is sound or not, and whether your ideas make sense. It doesn't matter whether you are 12 years old and unknown or 65 years old and world-renown. The ideas are judged on their merits. You're both NPs, so there will be less need to follow cultural restrictions based on age (IMO).
Well, why would it be, though?At the end of the day, my friendship with my INTP will not be swayed by what users post on here.
I don't personally really care what type my friends are (and I enjoy the fact that, when I list my best friends, they're from all different pools of personality type). I might suspect an easier time with certain personalities than others... but the bottom line is my ability to connect with an individual regardless of what type he or she is.