Let's start with something that cannot be debated. Kiara is Simba's heir. That means that either his firstborn died, or that she was eldest. How to figure that out is fairly simple.
Male lions begin to get their manes at around a year old. This little guy is about eight months. Probably comparable in age to Simba. He's old enough to be on his own for a bit, old enough to leave mom and be shown the Pridelands, but not old enough to fight hyenas or hunt. He's still vulnerable to predators.
Now compare this eleven month old at Cedar Cove Feline Sanctuary to Kovu and Kopa. See the mane fringe that's different from the eight month old? Kopa and Kovu are older than Simba was.
This is further supported by Akril's