This is a confusing subject. At first it was commonly thought The Lion Guard took place post- Simba's Pride which would make Kiara an adult. The future queen part of the description wasn't suspicious in terms to her age, as it's possible she'd only become queen when Simba and Nala die.
Another reason it might have been assumed she's an adult is due to her voice actress Eden Riegel, who sounds like an adult Kiara:
However the book "Can't Wait to Be Queen" kind of shot down the idea of her being an adult. Starting with a temporary cover:
owever this was disregarded due to the various inconsistencies in Kiara's design,
But when the final cover was shown, it was basically confirmed she'd be a cub:
Also the reported lack of Kovu supports the cub theory as well. He's the next king and it would be hard for him to just not show up.
This would mean Simba's Pride would need a complete over haul for Kion to exist. (I wrote about how Kion could possibly exist in Simba's Pride.)
But then there's this Tweet by Khary Payton who will voice Rafiki answered a fan question by saying it takes place after Lion King 2. Given that Kiara is an adult at the end of the film, this is puzzling.
We'll have to wait until November to know for sure.