Not the author, and I did read the Silmarillion a long time ago, but I had assumed until now that Gandalf was an elf because he carried one of the elven rings.
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.
That's correct. Cirdan gave Narya, the ring of fire, to Mithrandir when he arrived.
"Take this ring, Master," he said, "for your labours will be heavy; but it will support you in the weariness that you have taken upon yourself. For this is the Ring of Fire, and with it you may rekindle hearts in a world that grows chill. But as for me, my heart is with the Sea, and I will dwell by the grey shores until the last ship sails. I will await you."
