Well, if it wasn’t, we would all be in trouble. It’s not plagiarism if you’re quoting yourself, and it’s addressing the question.
What it should make you vigilant to, of course, is whether the question you’d answered and the question you’re answering might need to be merged…
EDIT: No, it is considered self-plagiarism. See exchange between John Gragson and Jay Wacker, Quora policy architect, starting at https://productupdates.quora.com… , and culminating with:
You can reuse your own, so long as you block quote and attribute to the original answer. This is not a new policy. People don’t want to read the same answer again and again.