In the following article, the underlined 'did' refers to the word 'mature' in the same sentence. But, how about the word 'reach' in the previous sentence? Is it ok to say the word 'did' can also be replaced with 'reach' for having the similar meaning in the context?
The verb did substitutes for the main verb in the same sentence (mature).
Did can't substitute for a verb of the previous sentence (for example, reach in the sentence from the answer) unless it is used immediately after the full stop before another verb.
"I saw Ann yesterday."
"So did I. She looked a bit frustrated."