I've often thought it would be good if you could stay young and healthy for a long time. You could do all the things you dreamed of doing. Instead, when we get old enough to understand what life is about and what we should do to make our life a happy success, we're on the way out:) Eternity is too much even to think about, though. To cope with that, we would have to transform into a completely different type of being.
The question is whether we would develop super-human wisdom or just be an anachronism. Our brains develop mainly after birth according to what our senses experience and that would change over the centuries. Yet we cannot change our earliest experiences, so we could be like a time traveler from the past.
Just a thought - as you get older, they say your ears and your nose keep growing. If we lived for much longer than we do, wouldn't we start looking really, really weird?