By popular I mean somewhat known, so they should atleast have charted.
Some of the older ones I know of:
GOD's Joon Park was 29 years old
2ne1's Bom was 25, Dara was 24
Brown Eyed Girls' JeA, Miryo and Narsha were all 24
Girl's Day Sojin was also 24
Red Velvet's Irene and T-ara's Boram were 23