Youthful

characterized by youth; young.
of, relating to, or befitting youth:
youthful enthusiasm.
having the appearance, freshness, vigor, etc., of youth:
she is 60 but her optimism has kept her youthful.
in an early period of existence; early in time.
physical geography. (of topographical features) having undergone erosion to a slight extent only.
contemporary examples

lenny kravitz’s ‘black & white’ alb-m marlow stern august 29, 2011
how pope francis became the world’s bff jason berry december 20, 2014
what rand paul aide jack hunter and his resignation say about his boss james kirchick july 22, 2013
this republican loved taxes & modern art scott porch november 18, 2014
this week’s hot reads: may 13, 2013 mythili rao, sarah stodola may 12, 2013

historical examples

in the mayor’s parlour j. s. (joseph smith) fletcher
bidwell’s travels, from wall street to london prison austin biron bidwell
the business of life robert w. chambers
little prudy sophie may
history of ancient art franz von reber

adjective
of, relating to, possessing, or characteristic of youth
fresh, vigorous, or active: he’s surprisingly youthful for his age
in an early stage of development: a youthful culture
also young. (of a river, valley, or land surface) in the early stage of the cycle of erosion, characterized by steep slopes, lack of flood plains, and v-shaped valleys compare mature (sense 6), old (sense 18)
adj.