Board Thread:Episode Discussion/@comment-27059722-20181114224948/@comment-40208359-20190730021924

They probably decided to age up the Guard members and make a short time jump because Kion and Bunga's voice actors' voices have broken and matured. If you compare Kion's voice in Return of the Roar with Fire From the Sky then you can definitely hear that his voice has dropped a lot. Bunga has too, but it's far more subtle and his voice actor has been able to adapt to keep his voice pitched enough to fit the character. Max Charles (Kion's voice actor),however, sounds strained and his voice is failing to keep like how it was at the start of the show, which is understandable since he was like 12 when he started voicing Kion and is 16 years old now.

So aging the guard up makes the most sense since Kion's voice stood out far too much compared to the others, since (I think) the other cast members are older than Max Charles and don't  have to worry about their voices breaking/changing.

Plus, they probably wanted or planned to age the characters up since they have to bridge the gap between the series and The Lion King II; Simba's Pride, explaining where Kion and the gang are and why none of the characters appear in other canon sources before the series.