MYSTERY SOLVED(?) [a thought after a week of finishing of just this season and did not watch other seasons]
in the episode of after zhang bu xun got poisoned by corpse poison just for holding part of the corpse in the misty village tomb, believe that zhang bu xun & zhang qing ping managed to have a baby (got mutated with corpse poison from zhang bu xun's poison) during zhang bu xun & zhang qing ping's stay in the tomb which zhang gao yuan & zhang qing ping found out that zhang qing ping got pregnant after they left the tomb when zhang bu xun died and so that's how their descendants (including zhang qi ling) was able to look young no matter how many years have passed as the mutated blood got carried on
in the episode of after zhang bu xun got poisoned by corpse poison just for holding part of the corpse in the misty village tomb, believe that zhang bu xun & zhang qing ping managed to have a baby (got mutated with corpse poison from zhang bu xun's poison) during zhang bu xun & zhang qing ping's stay in the tomb which zhang gao yuan & zhang qing ping found out that zhang qing ping got pregnant after they left the tomb when zhang bu xun died and so that's how their descendants (including zhang qi ling) was able to look young no matter how many years have passed as the mutated blood got carried on