So as it turns out, your king automatically ages to 16 when you retire him, no matter how old he was. That's confusing. So it means, your lion can't die young anymore, and his page will show wrong age forever. (I'm saying this as that kind of player who treats their lions more as characters than just breeding material. For example, my previous kings' untimely death was a part of the pride's story and source of drama xD) Anyway, you can retire your grandpaw anytime you want, so why not just keep their actual age?

UPD: Really: there's nothing in it (grandpaws keeping their age) that could harm the game's balance. Grandpaws can't breed, after all. Plus there can only be one Granpaw per den anyway. Also, one of the main points of Grandpaws was letting your king live longer, but in the end, having them aged up to 16 isn't much nicer than just "killing" them. I see absolutely no reason why they couldn't stay in their actual age.