Coin Age is a property of each UTXO; it's time that has passed from the moment of a UTXO's creation.
Every time you successfully stake, you get a reward for keeping the network secure. When you create a stake, you spend your UTXO with value X to reset the Coin Age, and in the same transaction, you get X + Reward to the same address, which creates a brand new UTXO and thereby resets your Coin Age.
To solidify the network stability, staking rewards (coins created by staking transaction) are unspendable for 20 blocks. These unspendable coins are considered to be "immature."