en_tn/luk/01/18.md

How can I know this?

"How can I know for sure that what you said will happen?" Here, "know" means to learn by experience, suggesting Zechariah was asking for a sign as proof. Alternate translation: "What can you do to prove to me that this will happen?"

my wife is advanced in her days

This is a euphemism that says that she was very old. See how you translated similar words in verse 7. Alternate translation: "she has lived many days" or "she has lived many years" or "she is very old" (See: rc://en/ta/man/translate/figs-euphemism)