To add to the above - always give yourself something to strive for, a goal (or goals) of some sort. Yes, as we age, depending on genetics and such, our bodies slow down, but that doesn't mean we have to stop.
My grandmother always told me to strive to learn something new every day - there's so much out there in the world, we can't learn it all before we die. Try things you think would be interesting - who cares if you are "good" at it or not - that attempt is what counts...as well as the fun and sense of accomplishment when you achieve it.
Travel, dance, sports, exercise, cooking, reading, internet searching, games, nature, create your own business, volunteering, ... the list is endless.
Another thing I found personally - have some activity you do that includes children - kids are full of life, and just being around them makes you feel full of life too. I got into figure skating, and there are lots of kids in that sport. I know I'll never be a Michelle Kwan, but I've learned to do things on skates I never thought I'd ever do. I found taking classes with kids helped find the kid in me - because, let's face it...if you are playing "skate tag" with a 6 year old - there's no way you're going to let them "win"! (hehehe)
Never stop dreaming, or creating, or learning. If things happen that challenge you physically - there are still options. Look at para and quadrapalegics - they are artists, lawyers, writers, etc - so having physical limbs doesn't mean you can't be creative and strive to learn and grow.
Another example - Jack Lalanne - very well known fitness advocate, recently died this week; but what an example of keeping yourself feeling young and fit he was. Maybe check out some of his old videos for inspiration!