The euphemism for "old willy"? :)

elderly people

There's not really a polite euphamism in English. Generally, we just show respect by addressing them a s sir or mam but that's not necessarily for old people, or just by being polite and respectful in the way we communicate. Americans are pretty relaxed about these things. You can refer to them in a general way way as elderly, aged or senior (adjectives) but you wouldn't use these to address someone directly. There are plenty of impolite ones, though we don't generally use them directly. Some are listed here: https://www.thesaurus.com/browse/old%20person

Excuse me father can you cam in here