First a puppeteer and then a clown, what next the whole circus?
Who knows, Frieren herself might be considered a youngling by that world's elven standings. But we don't really have that many reference points, since we've only met three elves aside her. The old as magic Serie, the priest Kraft who was around when gods still walked on earth and then the prankster, who did the beer prank during some ancient empire's haydays.
So who knows, 200, 1000, both might be basically adolescent in terms of elven ages. Take Tolkien elves, some are youngsters at few hundred years, while at the same time there is Cirdan, who was born before the Valar discovered the elves.
So anyhow, Olivia per human terms is old, but we don't have an elven reference point for age yet. So she might as well be in her elven 20s for all we know.