Unless you all are aware that the lion guard is a Miquel to the lion king 2, so this is based on after Either Rafiki's retrieving Mufasa's message with kovu and Kiara together or the villain song my lullaby.
http://ideas.wikia.com/wiki/The_Lion_King_IV I mean, Mheetu, Bhati, Kopa, Kovu, Kiara, Timon's Mother, Uncel Max, Rafiki, Zazu,Vitamin, Pridelanders, Outsiders (also known as Outlanders), Meerkat Colony, Animals,Twiga, Tiifu & 'Zuri will join the members of the Lion Guard for Kion, Bunga, Fuli, Beshte, & Ono to save Simba, Nala, Timon, and Pumbaa and return to the Pride lands before Von Drusko wipes out the entire Pride. right?
The lion guard is set in the time skip of the lion king 2 so before Kiara has grown up and attmepts her first hunt. There is no reason to change her age. What might need to changed however is Simba's voice and how Kiara currently acts as it doesn't exactly match up with how she was at the start of the lion king 2 and the end of it. She seems to actually want to be queen now which is a bit contradicting of her previous personality