An early reference describing these adepts is in the works of Paracelsus:
There are persons who have been exalted (verzueckt) to God, and who have remained in that state of exaltation, and have not died...
In the writing of the German mystic C.von Eckharthausen we find:
These sages, whose number is small, are children of light, and are opposed to darkness. They dislike mystification and secrecy; they are open and frank, have nothing to do with secret societies and with external ceremonies.- Disclosures of Magic (1790)
They possess a spiritual temple, in which God is presiding. They live in various parts of the earth, and do not meddle with politics; their business is to do as much good to humanity as in their power, and to drink wisdom from the eternal fountain of truth. They never quarrel about opinions, because they know the truth. Their number is small. Some live in Europe, others in Africa, but they are bound together by the harmony of their souls, and they are therefore as one. They are joined together, although they may be thousands of miles apart from each other. They understand each other, although they speak in different tongues, because the language of the sages is spiritual perception.
An historical example is Saint Germain. He was never seen to eat. (They do take liquids however.):
A master doesn't need solid food, and temperaments no longer influence or control him. He uses the choleric temperament to do his magic deeds, he lets the things of the physical world pass by him like a sanguine, he'll behave like a phlegmatic in his enjoyment of life and he'll brood about his spiritual findings and experiences like a melancholic. But it'll take us awhile to get that far, so we should try to bring our whole life into harmony with our spiritual striving.
- From the Contents of Esoteric Classes, Rudolf Steiner
We see also at the end of the Gospel of St. John that John, the Beloved "tarried":
Jesus saith unto him, If I will that he tarry till I come, what is that to thee? follow thou me.
Then went this saying abroad among the brethren, that that disciple should not die: yet Jesus said not unto him, He shall not die; but, If I will that he tarry till I come, what is that to thee?- John 21:22-23.
Christ was not talking of life beyond the grave. He was not talking of the resurrection. He was talking to Martha concerning Lazarus, who was already buried.And whoever liveth and believeth in me shall never die. Believeth thou this?- John 11:25
The bodies of the Elder Brothers, though appearing normal, are differently constructed to that of the bulk of humanity. There has to be a pattern set in place for the future and it is this they bear.
Some quotes from the B.Hive:
"Such men do not cease to be human. There are glamorous misconceptions which pronounce a Master to be so divine he is as separate to you or I; or so far removed that he is above the comprehension of the daily contests. However this is not the case, they have not the angelic withdrawal, and are sensitive to the individual stirrings and affectations of any man.
"Secrecy is a great protector. It has been in the past, that the respect is issued as is warranted, and details of movements and even heritage are uncalled-for.”
"Just as the royal family goes about with consorts to guard, it is the veiling attendants which move about the Masters, shielding them from undue arrest. Even we have regarded much of their business to be their business.
"It is as this also: The work of one Master is uniquely different in his approach and conduct to that of another - and this is of necessity we might add. But the two specific tasks are dependent upon the success of the other, and are in no way confused by the extreme distinctions."
"The true test of the teaching is of the teaching itself. No Master may claim infallibility. For this would negate all possibility of change.
“'Master' is a relative term - and implies also, one who has trod the boards at an earlier period - and therefore is relative only to time, and not to quality of super-character, or pompous title. Duty performed is different to duty inherited."
end of quote
"Of a Master it is said: He is born in the same body; he uses it for hundreds, even thousands of years. This is the case with the vast majority of leading Individualities. An exception is formed by certain Masters who have their own special mission; with them the physical body remains, so that death does not occur for them at all. These are the Masters whose task it is to watch over and bring about the transition from one race to another."
-Rudolf Steiner, Theosophy of the Rosicrucian, page 54: There is more on page 53