From a business standpoint, anyone 23 or over would not be considered a sound long term investment nowadays. While they are still very much capable of performing, some may be considered as "over the hill" in an industry where new 4th/5th gen groups are now being pushed with 14 yr old early teens.
Times have changed, sadly. Which is probably why, personally, i'm not into Kpop as I once was. Back in my Teens, Minzy was the odd exception in the biz and most idols were on the verge of 18 when they debuted. Those days are gone.