Use the adjective youthful to describe someone who is young or who acts as though they are — like your surprisingly youthful grandma, who just signed up for a hiphop dance class and rides her skateboard around the neighborhood.
This word is handy for talking about the general characteristics of young people, like youthful energy, youthful stubbornness, or youthful enthusiasm. You might also say someone looks youthful if they appear younger than they are. Someone who acts youthful might enjoy swinging on the swings at the playground, even if they're no longer a kid. At the heart of youthful is youth, "the period between childhood and adulthood."