I just noticed that the older lady noticeably grows up too. She's even going to the same school as the younger one in the first two pictures. My guess is a 4-5 year age difference.
Most of the girls in Kemono Friends, other than a few whose ears are covered (like Gray Wolf) or their animal doesn't have visible ears anyway (like Shoebill), have four visible ears. It's kind of weird, and opens up all kinds of fan speculation about how their hearing works.