It is debatable whether any kind of age can be placed on Olórin, the ainu who became Gandalf. It seems that time of some sort existed in the Void where he was first created, since events progressed and changed; but at the same time, the void is also called the "Timeless Halls", and the ainur who entered the world did so "at the beginning of Time". So, from an earthly perspective, Olórin is as old as the world/universe.

Exactly how old that is in terms we would understand is unclear. The Silmarillion tells that the Valar and the Maiar laboured to build the world

in wastes unmeasured and unexplored, and in ages uncounted and forgotten, until in the Deeps of Time and in the midst of the vast halls of Eä there came to be that hour and that place where was made the habitation of the Children of Ilúvatar

So really, we can't put an age on Olórin, or any of the Valar or Maiar.

Gandalf, on the other hand, the embodied form of Olórin who walked Middle Earth to oppose Sauron...him we can date with reasonable precision.

From Appendix B of Return of the King, The Tale of Years, regarding the Third Age:

When maybe a thousand years had passed, and the first shadow had fallen on Greenwood the Great, the Istari or wizards appeared in Middle Earth. It was afterwards said that they came out of the Far West and were messengers sent to contest the power of Sauron, and to unite all those who had the will to resist him; but they were forbidden to match his power with power, or to seek to dominate Elves or Men by force and fear.

Exactly when Gandalf arrived is, as far as I have been able to find, nowhere directly stated; we know that he was the last of the Istari to arrive, and seemed the least impressive; we can assume something like the years 1000-1025 TA. He finally left Middle Earth with Bilbo and Frodo in 3021 TA, so I choose to believe he arrived in 1021 and stayed for exactly 2,000 years.

It's also worth noting that he never appeared young; none of the Istari did.

They came therefore in the shape of Men, though they were never young and aged only slowly

This 'aging' appears to have been related to the trials and difficulties they had to endure, rather than to time.