I have a few questions regarding the Greybeards. There probably aren't any real answers (otherwise I would probably have been able to find them) so I welcome any insight or thoughts you have on the topics.

1. How old are the Greybeards... I mean the individuals themselves and not the order. Obviously they are old but how old exactly? Are they only eighty or have they prolonged their lives and are maybe centuries?

2. How many Greybeards are there? Are there only the four or so that we see in-game or like many aspects of the games is it a scale issue and there are more members that we do not see.

3. Concerning tutelage and new disciples. How often do the Greybeards take students? One a decade? A year? A century? How many fail their learning?

1. We don't know obviously old enough to have greybeards, I also doubt that they use any magic to prolong their life at least intentionally.
2.It's likely that this is the case but even then number of greybeards shown is propoboly closer to reality then number of lets say citizens of whiterun.
3.We know that Ulfric trained to be a greybeard before great war, we albo know that Jarl of Whiterun tried to join them if I remember correctly.

1. Well the Greybeards were founded sometime in the first era, but how old the ones we see in Skyrim is unknown.

2. Its fair to assume that in lore they might be a bit bigger, but for now those four are the only greybeards.

3. Ulfric trained with them. Balgruuf only climbed the steps but there is no evidence or clue that he actually trained with them like Ulfric. i think they take new disciples if they see them fit enough, doesnt have anything to do with time. and i assume a lot do fail. even Ulfric failed and never completed his training.