In-game currency would integrate with a few of my past suggestions (main menu shop, treasure chests, loot boxes, etc). I think the in-game currency should be called “age coins” or something like that. You would earn 1 age coin every year of a life (as a result, you can earn a max of 60 age coins in 1 life). Through in-app purchases, age coins could also be bought for real life money from the main menu. The bigger the pack of age coins, the more real life money they would cost.
I think that 1 age coin could look something like this. From there, you get the idea.