Now ya'll know how much I love Jinny. She would definitely fit the group and her personality is amazing. However if she doesn't yet have the skills and needs more time to train, then it's probably best she isn't put in this group.
Plus having a maknae with a fun personality isn't always a must. Think of SNSD, their maknae is known for being quite timid and mature, while their variety queen (Sunny) is the second oldest! Same with APink where Bomi is also the second oldest while the quieter Hayoung is the maknae. And of course in 2NE1 the two dorks of the group are the two oldest members.
As long as the group has one or two fun members, it doesn't really matter what age they are.