The only way I can explain what I want to know is by example.
So, lets say I create a wallet. Buy 20 coins on Monday, then 30 coins on Tuesday and then 50 on Wednesday.
Later in the week, on Friday, I decide to tip a friend 10 coins.
So, on Friday I have 20 coins that are 4 days old, 30 coins that are 3 days old and 50 coins that are 20 coins old. When the tip is send to my friend I will end up with 90 coins. Which coins are those leaving my wallet? The younger ones or the older ones?
I am asking this as I read somewhere that the coin’s age does matter on staking. (I am not 100% sure about that but I would love if you could verify this. If this is not true then what I am asking doesn’t really make sense.)