young fogey

bright young thing

One who is youthful, clever, eager, and high-spirited in manner and attractive in appearance.

old fogey

An old and overly conservative person.

old head on young shoulders

Alternative form of good head on one's shoulders.

sweet young thing

An attractive young woman.

young at heart

Having the joy and vitality associated with younger people, despite one's own advanced age.

young blood

Young or youthful people, especially as a source of revitalizing force (in a team, organization, etc.).

young Turk

A young person who agitates for political or other reform; a young person with a rebellious disposition.