Younger is probably easier. I was discussing this with one of the senior members of our ham club, and he told me, "The best way to learn Morse Code is to start when you're nine years old". I told him, "Thanks for the tip. I'll be sure and do that next time around". But in this life, for me, learning it at nine years old isn't one of the available options. On the other hand, I don't know of any better way to keep a mind young than to continuously learn new skills. Don't get set in your ways. Stretch your mind by doing something hard, different, a bit outside of your comfort zone. And if you are one of those who already learned CW at the age of nine, consider learning to play the piano, or to speak a foreign language, or to juggle, or paint, whatever. You may not become as good at these tasks as someone who started so early, but you can be good enough to have fun, and to keep your brain active.