Is "at" necessary in "how and at what"? I don't understand why "at" is being used in that phrase.
Yes, "at what" is grammatically correct here. It has been used to mean "when (age)," and can also be substituted with "when" here without changing the meaning of the sentence. Ex: Who and where are we meeting? Ex: How and at what age we die depends on a number of factors.
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