In Episode 3, Yoda is teaching Padawans of roughly the same age: around 6. Is there a rule or principle that Jedi teach Padawans of the same age?
There is a well-defined logic behind taking children as Jedi.
The Jedi council prefers Jedi who do not have emotional attachments. Older children generally get attached to their family or even fall in love. This is the same reason the council was skeptical to have Anakin as a Jedi.
Now one would argue that Luke was much older when he went forth to become a Jedi. Well, for one he wasn't emotionally attached to anyone or anything, living on an isolated planet. Also, the Jedi council didn't exist so, no one cared.
There is a reasonably well upvoted non-canon answer here stating that it appears from the movies that Padawans start very young to prevent too many attachments to the lives they will leave behind. Given this, it would make sense for there to be a "beginner's" class for the younglings who have only recently been brought to the Jedi Temple. Since they were always brought in at a young age, to have a group of more or less the same age seems logical.
The older Padawans would have quite likely been assigned to a Jedi Master to learn from "in the field", so to speak, and may not train specifically in the Jedi Temple any more.
It's a common form of pedagogy to train students of the same age under the presumption that they will all learn at the same rate. It stems from the design of the various municipal school systems which were born during the industrial revolution and children are treated as a manufactured product. No school will ever quite match the quality of an old fashioned one on one experience. However, there were many children to train and the Order had a lot of universe to police. Going this route allowed them to churn out Jedi at a more rapid rate.
However, one wonders how many of those students achieved any sort of real mastery over the force. Why are there so few masters? I believe much can be blamed on the educational system they had. Perhaps if there were fewer Jedi who were more individually trained, the whole thing would never have happened.
Now the Jedi clearly knew that they couldn't entirely stamp-manufacture Jedis, and as a result all of the basics of Force mastery were taught in classes. However, padawans were still required to go through an apprenticeship before they would be allowed to operate on their own, to make sure that they were all up to a minimum level of competence.