I would like to have the best odds that in 30 years, when in my 80s, i can balance and walk on my own. what activities, exercises will help me best reach this goal?

Prevention: Great idea to work on prevention rather than waiting until there is an issue. Life has no guarantees and isn't always fair but we can lower the probability of illness w/preventive steps. Make sure all is well physically by being followed by your doctor, consider some sessions w/a trainer at a gym to get a tailor made program for you. Positive psychology books. Peace and good health.