This aging appears to have been related to the trials and difficulties they had to endure, rather than to time. As you can see Gandalfs real age is very hard or impossible to determine, but his age as a mortal man in Middle-Earth is somehow possible to determine, and it would be somewhere around 2,019 years.
Why is Gandalf immortal?
As one of the Maiar he is an immortal spirit, but being in a physical body on Middle-earth, he can be killed in battle, as he is by the Balrog from Moria. He is sent back to Middle-earth to complete his mission, now as Gandalf the White and leader of the Istari.
Why is Gandalf an old man?
Gandalf is received by Cirdan Olórin, like the other Wizards, took the shape of an old man. At Mithlond he was welcomed by Glorfindel, his friend from Valinor (who had been sent earlier on a similar mission), and Círdan the shipwright, who possessed Narya, one of the Three Elven Rings of Power.
Is Gandalf actually old?
Gandalf walked in Middle-earth for 2,019 years, un-aging and appearing as a grey-bearded human of about 60 years. But as a Maia, he lived at least 9000 years before Middle Earth even existed. That means his real age is close to 11,000 years.