1. He is past old age, so he is very weak. He is about to pass away.

Question

2. He is past middle age, so he has to work hard to prepare for old age.

3. He is past young age, so he is able to cope with difficult tasks.
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Are they all grammatical? Would yoiu check them? Thank you.

Answer 1

#s 1 and 2 are fine.

#3 is phrased strangely. Try this:

He is not in his 20s anymore, so he should be able to handle difficult tasks.