suru verb (special), intransitive verb
be about to reach; be approaching (an age, a milestone)
Literary expression used when a certain age or point in time is imminent. Often appears as 〜歳になんなんとする or similar constructions. The kanji 垂 suggests something hanging down or impending.
See also: 垂んとす
彼は三十歳になんなんとしている。
He is about to turn thirty.
春も終わりになんなんとする頃、花が散り始めた。
As spring was drawing to a close, the blossoms began to fall.