But facts are facts: the Electress Sophia of Hanover was first in line to the British throne from the moment of Queen Anne's accession to her own dying day -- never becoming displaced in the succession. As it was, she was replaced as heir not by any child of Anne (whose pregnancies all preceded her reign) but by her own son, the Elector George of Hanover (who would become King George I of Great Britain).

I suppose, if one is talking of heirs presumptive in royal history, there would actually be many elderly persons: after all, one can belong to an older generation to that of the reigning monarch.
And the list includes both those who did succeed to thrones and those who did not.

Sophia's position in Great Britain was not unlike that later on of Prince Luitpold in Bavaria, who was heir-presumptive to his nephew Otto (who had succeeded his older brother, Ludwig II, as king), from the day of the latter's accession (in 1886) until his own dying day (in 1912), when he was replaced by his son, who would eventually succeed to the throne as King Ludwig III. The difference was that the latter deposed his mentally disabled cousin and assumed the throne within his lifetime -- the day after the Bavarian parliament passed a law enabling him to do so.

Luitpold, who served as regent of the kingdom for the last 26 years of his life, probably never expected to formally become the official sovereign. As it was, he lived to an even more advanced age than Sophia: so at 91, perhaps he could count as the oldest heir ever in European royal history.