记得要给上下方啊，否则任何人来看也是一头雾水啊。

At the age of twelve years, the human body is at its most vigorous. It has yet to reach its full size and strength, and its owner his or her full intelligence; but at this age the likelihood of death is least.

has yet to是指尚未

and its owner his or her full intelligence是省略表达，还原后为

and its owner has yet to reach his or her full intelligence

its owner 指身体的owner，当然是人了。