An immortal man
The musican Rameau and Madem de Gergy both assert that thy met Saint Germain in Venice, Italy in 1710. under the name of 'Marquis de Montferrat'. He had an apperance of forty to 50 years old. This evidence destroys the hypothesis that Saint Germain was the son of Marie de Neubourg or Francis Racoczi II, for if he had been, he would have not been more than 20 years old. Saint Germain was described as 'a man of middle height, strongly built and dressed with superb simplicity.'
The Period of his great celebrity in Paris extended from 1750-1760. Everyone agreed then that his apperance was between forty and fifty. Saint Germain disappered for 15 years and when the Comtesse d' Adheman saw him again in 1775, she stated that he looked younger than ever. When she saw him 12 years later again, he looked the same. "I have seen Saint-Germain again," wrote Comtesse d'Adhemar in 1821, "each time to my amazement. I saw him when the queen was murdered, on the 18th of Brumaire, on the day following the death of the Duke d'Enghien, in January, 1815, and on the eve of the murder of the Duke de Berry."
"He diluted the strength of the marvelous in his stories," said his friend Gleichen, "according to the receptivity of his hearer. When he was telling a fool some event of the time of Charles V, he informed him quite crudely that he had been present. But when he spoke to somebody less credulous, he contented himself with describing the smallest circumstances, the faces and gestures of the speakers, the room and the part of it they were in, with such vivacity and in such detail that his hearers received the impression that he had actually been present at the scene. 'These fools of Parisians,' he said to me one day, 'believe that I am five hundred years old. I confirm them in this idea because I see that it gives them much pleasure -- not that I am not infinitely older than I appear.'"
In 1784 Saint Germain claimed that he was tired of life. He seemed careworn and melancholy. He said he felt feeble but refused to see a doctor. There are no details that exist of his death, or rather of his supposed death. No tombstone at Eckenforn bore his name. Even tho he was known to be dead after that, he was seen again in 1821.
What I think happened
I think Saint Germain was just a made up tale, or multiple men were mistaken to be Saint Germain.