Solar and Moonbyul are both over the age of 25 and still sold well with their solo albums. The average age of the group is around 26-28 but Dingga still performed decently on charts.
For a different example, Lee Hyori is like 40 and is still doing well. Taeyeon is 32 and still one of the most relevant female idols right now. Hyuna is 28 and doing alot better with I'm Not Cool compared to Flower Shower.
I feel like this mentality of female idols having to be a certain age to be successful is slowly dying off as KPOP advances.