When you freeze a lion, they stay the same age forever. Seems simple enough, right?
Well, I happen to have a frozen lion. And I found that even though you can't see their age, the frozen lion is still sorted according to it... which can end up placing them behind other, normal lions in the age sort.
This brings us to my proposal - frozen lions being sorted as if they were the oldest lions in their age group. This would help with logical age-based sorting, and here's an example:
Let's say we have three lions - an adult, an adolescent, and a young adult frozen lion. Normally, the order would be: adult, frozen lion, adolescent. Even though you might have had the frozen lion for longer, it's still technically not as old as its fellow adult. But using this method, the lions would be sorted in this order: frozen lion, adult, adolescent. This makes more intuitive sense, and I feel that it would make the game a better place.

-Edit-
To clarify, "age group" refers to the visual age of the lion. If you look at the stage bar on a normal lion, each of those segments is an age group. Frozen lions don't have the stage bar, so no visual changes would be made.