You have no interest in being 21 again. (Neither do we.) But, oh, wouldn’t it be nice to feel 21 again: The energy! The metabolism! The sense of I-can-accomplish-anything-I-set-my-mind-to! “It’s totally possible to rediscover that zest and optimism you felt when you were younger,” says Caroline Adams Miller, author of "Creating Your Best Life." “In fact, recapturing those qualities is essential to leading a healthier, happier life in the long run.” Take these simple measures to turn back your internal clock and make the date on your driver’s license feel like a big fat lie.