In fiction, it's easier to explain a younger looking person being older within the storyline than their actual IRL selves. It's far harder to do that the other way around.the same way you explain steward and picard having a ten year difference in age. you don't.
Modern Genetics have many people look younger than their actual age and it's pretty much guranteed that future people will end up looking younger and younger while being technically older thanks to numerous enhancements to appear and function at a younger age.
The search for the "Fountain of Youth" is endless by many folks and I doubt it will subside.
The quest for "Youth" and "Immortality" are endless searches that countless folks and fiction has been based on. But with future medical treatment and who knows what other "Genetic Enhancements" can do for the masses.
Imagine if a specially coded genetic marker would allow "Differential Aging" and slow your internal body's organs to age slower alongside outward physical aging relative to current 21st century standards so that you live a more enjoyable and functional life after puberty ends.
Imagine if this was your "Differential Aging" relative to 21st century human standards.
_22 y/o that looks like a 19½ y/o with the vitality of a 18⅔ y/o if you slowed your aging @ 17 y/o
_24 y/o that looks like a 20½ y/o with the vitality of a 19⅓ y/o if you slowed your aging @ 17 y/o
_26 y/o that looks like a 21½ y/o with the vitality of a 20 y/o if you slowed your aging @ 17 y/o
_29 y/o that looks like a 23 y/o with the vitality of a 21 y/o if you slowed your aging @ 17 y/o
_32 y/o that looks like a 24½ y/o with the vitality of a 22 y/o if you slowed your aging @ 17 y/o
_36 y/o that looks like a 26½ y/o with the vitality of a 23⅓ y/o if you slowed your aging @ 17 y/o
_40 y/o that looks like a 28½ y/o with the vitality of a 24⅔ y/o if you slowed your aging @ 17 y/o
_45 y/o that looks like a 31 y/o with the vitality of a 26⅓ y/o if you slowed your aging @ 17 y/o
_50 y/o that looks like a 33½ y/o with the vitality of a 28 y/o if you slowed your aging @ 17 y/o
_56 y/o that looks like a 36½ y/o with the vitality of a 30 y/o if you slowed your aging @ 17 y/o
_62 y/o that looks like a 39½ y/o with the vitality of a 32 y/o if you slowed your aging @ 17 y/o
_68 y/o that looks like a 42½ y/o with the vitality of a 34 y/o if you slowed your aging @ 17 y/o
_75 y/o that looks like a 46 y/o with the vitality of a 36⅓ y/o if you slowed your aging @ 17 y/o
_83 y/o that looks like a 50 y/o with the vitality of a 39 y/o if you slowed your aging @ 17 y/o
_92 y/o that looks like a 54½ y/o with the vitality of a 42 y/o if you slowed your aging @ 17 y/o
102 y/o that looks like a 59½ y/o with the vitality of a 45⅓ y/o if you slowed your aging @ 17 y/o