over the hill

over the hill
no longer able to do something at an acceptable level because of age past your/its prime 

Some judges who are 75 may be over the hill, but others still have energy galore.

You're only fifty! You're not over-the-hillyet. 

Some people seem over-the-hill at thirty.

Some people say that singer is over the hill, but I think her voice gets better with age. 
盛りを過ぎたという人もいるが、彼女の歌声は年齢とともに深みを増していると私は思う。

He's over the hill.
彼は盛りを過ぎている。

He that stays in the valley, shall never get over the hill.
谷間に残るものは、丘を越えることはない。
現状に甘んじていれば何も達成できない。