I get tired so easily these days.
我最近总是感到乏力。
That means you're over the hill.
那是因为你在走下坡路。
Now, he is over the hill.
现在，他在走下坡路。
over the hill
走下坡路

If I'm too old to cut the mustard, maybe I should retire.
如果年纪大了不能胜任，我可能就会隐退
Cut the mustard
胜任
He can't cut his mustard.
他不能很好地完成分内工作。
If he can't cut the mustard, we'll have to find someone else to do the job.
如果他没能力完成自己的工作，我们会找他人来做。