young/old for one's years

:  being in the first or an early stage of life, growth, or development

:  junior

:  of an early, tender, or desirable age for use as food or drink

:  young persons :  youth

:  a single recently born or hatched animal

:  dating from the remote past :  ancient

:  persisting from an earlier time

:  of long standing

:  the period of about 365¹/₄ solar days required for one revolution of the earth around the sun

:  the time required for the apparent sun to return to an arbitrary fixed or moving reference point in the sky

:  the time in which a planet completes a revolution about the sun