Life extension is scientifically possible, as far as we know. There’s nothing to rule it out, anyway. Modern medicine has already improved average lifespans from just a century ago. Old age is actually programmed into our DNA, to make room for later generations. If they can isolate and “turn off” the aging gene (although I’m sure it’s not that simple), humans could potentially live for centuries. Then we could reach other stars within a human lifetime. But if not, multigenerational starships are not unknown to science fiction.
“Ok what living organism lives centuries?”
Trees.