速度界限 meaning in English

speed envelope

I recently turned fifty , which is young for a tree , midlife for an elephant , and ancient for a sportsman , fifty is a nice number for the states in the us or for a national speed limit but it is not a number that i was prepared to have hung on me
我最近转五十,对一棵树是年轻的,中年对于一只象,和对于一位运动家是远古的,五十在美国是州的一个美好的数字否则为国家的速度界限但是它不是一个我准备的数字已经在我之上吊。