Is the author doing hypothesis by using the form that I have marked? This is the opening sentence of a ghost story which is set in Isezaki.
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The "volitional" う/よう has several meanings. Here, these なろう and あろう express a guess of the speaker (i.e., "it seems ～", "I think ～", "look like ～"). Basically they mean the same thing as もう20年にもなるだろうか and 覚えている人があるだろう, respectively. This usage is uncommon in spoken Japanese, but not rare in literary and stiff sentences. You may have seen
adjective + かろう before, which is essentially the same pattern.
As for the second sentence, ある is an old-fashioned and stiff way of saying いる. In archaic Japanese ある was safely used for people (e.g., 人あり in archaic Japanese = 人がいる in modern Japanese). Normally this sentence should be written as 覚えている人がいるだろう or 覚えている人がいよう.