Exuberance! That's it - it stops us becoming fusty and dull in our 'mature' years.
I know and admire two people who epitomize what I wish to be like in my 70's. They are both passionate about their hobbies and interests, and aren't shy to share their exuberance when learning new things. Their unbridled joy and passion spreads enthusiasm to all around them. It keeps them both so young and alive, and vital.
It's what I'm aiming for!
Thoreau stumbled upon this wisdom a long time ago, when he declared: None are so old as those who have outlived enthusiasm.