The words "old" and "well advanced in years" mean the same thing. The two of them together emphasize that Abraham was "very old." (See: [[rc://en/ta/man/translate/figs-doublet]])
Abraham was about to ask the servant to swear to do something. Putting his hand under Abraham's thigh would show that he would certainly do what he would swear to do.
## and I will make you swear ##
This can be expressed as a command: "and swear."
## swear by Yahweh ##
This can be translated as "promise me in the name of Yahweh" or "promise me with God as your witness."
## the God of heaven and the God of the earth ##
"the God of heaven and earth" or "the God of everything in heaven and earth" (See: [[rc://en/ta/man/translate/figs-merism]] )