Like April (a 3rd gen GG), they have one adult member in their 20s, and the majority of the others are underage. People found it odd, so what happened to April was that the oldest member was transferred to a completely different group, KARD (3rd gen co-ed group).
This probably won't happen to Baby Monster, but compared to the 3rd gen more people care about age now, as often discussed regarding IVE and NewJeans.
Ruka is old enough to do mature concepts like BPink. But the 5 other underage members would just be able to do high teen YG style. And even the maknae might be too young for high teen. So YG can only do a compromise by making the younger members seem 'older' and the older member seem 'younger' to fit the concept, but this could cause problematic issues.