Along 是状语
came 是谓语
a youngster of five是主语,这是完全倒装句
headed for the dispensary是过去分词短语 作状语 ,
stepping high with the consequence of a big errand是现在分词短语作状语,
possibly one是同位语
to which his advancing age had earned him promotion是定语从句 修饰the one.