There is much to think here Molly, but first to say something briefly. From reading the quote it was clearly a quote of an older person. It is not an accident that it was written when he was already 81 years old. Artists, and maybe all of us, return to youthfulness and childhood in old age. For example, Woody Allen's latest movie feels like a young Woody Allen movie. Not completely, it still has the pace of an older person, the mastery of someone who has already done many films, but it also burst of youthfulness and thus is very different from many of his middle-aged movies.
The same can be said about Bernardo Bertolucci (Stealing Beauty, The Dreamers).
The same cannot be said about many others.

That is, to say we will die young also means that we will die old and in a good mood. Middle aged people are not young. Perhaps the reason is that the mastery allows you a simplicity which resembles a young age. Matisse late works could have been thought to have been done by an 8 year old (supposedly), as he was searching for that simplicity. (I'd need to think more about this but wanted to answer.)
Here's a Matisse painting from 1953, (84 year's old):

At least in visual art, age does not seem to be responsible for greatness,  passion does. 

There have been no lack of artists who demonstrated great authority and created ageless works of art at very young age, Egon Schiele was in his 20's when he died, and Franz Marc was in his 30's ... When Corot was dying as an old man, his last words to his bedside companion, Alfred Robaut, were:"You have no idea what new things I realize I can do. I glimpse things I have never seen before. It seems to me now I never knew how to paint a sky. What I see before me now is much more rose-colored, deeper, more transparent. Ah! how I'd like to show you these endless expanse of sky ...", his hunger and passion for art never faded even when his body was giving away.

Maybe what Matisse means is to be able to retain a sense of wonder that is so typical of children. The art of looking at everything that one has seen under the sun again and again after many years, and can be looked at freshly with the wonder of a child is probably every artist's (or person's) wish. It would be so nice to remember seeing/smelling a rose for the first time, or hearing a great master-piece for the first time, as Freud writes: "Infantile feelings are far more intense and inexhaustibly deep than are those of adults".

It is a subject that occupies me- how to play pieces and work on them hundreds of hours and play them freshly as if they were just written and I'm just discovering them.
I also highly recommend to see Terrence Malick's last movie "The tree of Life". It's a great movie which deals a lot with the subject of wonder.