As the title asks, how old was Yoda when he attained the rank of Grand Master of the Jedi Council? (Canon answers greatly preferred) According to Yoda's Wookieepedia entry (emphasis mine):
Around the age of 100, Yoda was ready to pass on what he had learned. Having attained the rank of Master, he spent the next eight centuries training and tutoring generations of Jedi. It was estimated that over his lifetime, he trained around 20,000 Jedi.
So, this tells me that he was a Jedi Master around 100, but he certainly wasn't yet appointed as the Grand Master of the Council. Further:
During the invasion of the planet Naboo by the Trade Federation, Yoda served on the Jedi Council as the Grand Master of the Jedi Order, having trained Jedi students for about 800 years.
Another tidbit I found on the Wookieepedia entry for Grand Master states:
The title of Grand Master was reserved for the oldest and wisest living Jedi Master.
There are obviously other long-lived species and races that make up the Jedi Order, including another member of Yoda's species (Yaddle, who happened to be younger than Yoda) who was even on the Council, but, at least at the time of the Trade Federation's invasion of Naboo, Yoda was far and away older than any of the members on the High Council.
I simply can't seem to find a direct reference to really pin down when he actually took on the mantle of "Grand Master". Has he held that title ever since it was first granted, or, as might be indicated by the "age" requirement, has it passed between Masters as the years progressed?
As a part of the answer, while not "required", if there's any specific information regarding when Yoda was actually appointed to the Jedi High Council, that would be useful as well. Additionally, if there is a list somewhere of Jedi Grand Masters through the history of the Galactic Republic, that would be tremendously helpful.
EDIT (reference from Legends): So, the closest thing I've been able to find so far is, in the Legends section of the Wookieepedia entry for Grand Master (referring to the Jedi Order), there is a reference to "Notable individuals". There are several Jedi listed here, including Master Yoda, with what I assume are the dates of their tenure. However, many of these time span references do not include definite start or end dates.
I bring this up because the nearest predecessor mentioned is Fae Coven, who served as the Grand Master from approximately 990 BBY. The end of her tenure, however, is not specified, although, in the same Legends article for Grand Master, it states:
Master Coven served a long term, even meeting her eventual successor, Yoda. Master Yoda was the last Grand Master of the Old Jedi Order, holding the title as early as 82 BBY.
While this may be as close as I'm going to be able to get, as I stated above, I would prefer a canon source, if available.