In all seriousness, what I like best is the familiarity with myself. Knowing my body, its capabilities and its limitations. Knowing my mind and my temperament, how hard I can push myself out of my comfort zone and when I need to back off.
As a young kid I used to cop an attitude that "I don't care what anybody thinks of me!" Ha. Now I know what it's like to REALLY not care what anybody thinks, and it is grand.
Keeping my cool when I need to is way easier than it used to be. As a young person I used to struggle with anger a lot. And depression is another thing that just doesn't seem to trouble me anymore. Even into my 40s I would get into really bleak moods and have self-destructive thoughts. Now? Sometimes I get lonely, or I'm dissatisfied about one thing or another, but that kind of depression just doesn't happen anymore. I don't know if it's age (and, hopefully, maturity) alone that's done the trick, or if having Bunny in my life made the difference, but it's apparently a permanent change for the better.
I think about mortality plenty, but not in the mystified way I did when I was younger. It was something largely unreal then, and far-off - exotic and monstrous, yet abstract. Now? I'm thinking in concrete, pragmatic terms about it, and getting on with life. I've got a will made out, I've worked out a calendar for my expected age at various points in the future, and I'm going to be looking into buying a plot before too long. And other than that, I'm planning out the years I figure I've got left and gonna try to make the most of them.