How old were Edward and Alphonse when they performed human transmutation? I am referring to the Brotherhood series, not the 2003 version.
Brotherhood more or less follows the manga. I don't remember if we get a montage of events in the brothers' childhoods with their ages (a quick search through volume 6 doesn't give me anything), but we do see Mustang arrive at Resembool presumably shortly after their human transmutation attempt, so we can get some information from there.
So Edward is at most 11, and Alphonse 10, when human transmutation is attempted. This matches up more or less with the ages I remember possibly having seen from 2003 and Brotherhood. (Keep in mind that some chronology - such as the death of Winry's parents - might have occurred at a different point in the manga continuity compared to that of 2003, and that there is the additional half year before Ed takes the State Alchemist exams.)
In Episode 2, the past was shown which was 10 years ago.
Let's start when his mother was still alive.
Edward was 7 years old and Al was 6 years old.
Research and training (4 years)
When their mother died, Edward and Al agreed to revive their mother again by using the taboo which is the human transmutation, but before that, they searched about the human transmutation and improved their alchemy by training with the alchemy master in order to revive their mother...it took many years. After the training, they've completed the ingredients and performed the human transmutation.
Then that dark incident happens...
It took 4 years of preparation and then their bodies were the sacrificed...
When Edward became a state alchemist / test part (1 year)
After that "dark incident", Roy Mustang came to their town hearing that there is a talented alchemist which is referred to as Edward (knowing what Edward have done), then he offered Edward to become a state alchemist and told him about the Philosopher Stone that could bring back their body, which Edward grabs that.
Edward had his surgery and rehab which took only one year.
He was twelve years old then, and Al was 11 years old.
To shorten my answer, Edward Elric was 11 years old and Al Elric was 10 years old when they've done the human transmutation. (Refer to Brotherhood series; his present is 17 years old in Episode 2.)
In part 1, episode 6, Alphonse says that he is 14 and Edward is 15.
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