Gandalf's existence is actually quite interesting, he's certainly at least a Maia but may in fact be a bit more. There are some murky points about whether he's a devoted servant of Manwe and one of the eldest Maiar - or whether he is actually just an avatar of Manwe.
Do you have a source on which you are basing this conclusion?
From my perspective, it is pretty well established that Gandalf is Olorin, a Maia distinct from Manwe. The non-interference of the Valar in the Third Age is also well known so it is doubtful Manwe himself would appear physically in Middle-Earth.
Also, all Maiar (or rather, Ainur) are the same age or, more accurately, ageless, since they existed before the world.
