Hello ! First, I found out that 지긋하다 becomes 지긋이 but not 지긋히 when it is used as an adverbe. So you can say 나이가 지긋한 사람 or 나이를 지긋이 먹은 사람. Both of them. We use "나이가 지긋한~” or any other applied sentences to describe elders in a polite way. We don't use it to designate young people. And "나이를 먹다” is same as "나이가 들다” (grow old) Example: 나이가 지긋한 노신사 A gentleman of a certain age

지긋지긋 is a totally different word from (나이가) 지긋한, although they look alike each other. Adverbe of 지긋한 is 지긋이 Adverbde of 지긋지긋하다 is 지긋지긋 or 지긋지긋이 But l've never heard of the adverbe of 지긋지긋하다 in real conversation. I think we rearely use it while we use 지긋지긋하다 a lot.