Coin age is a concept in the proof-of-stake (PoS) system, referring to the amount of time coins have been inactive. The PoS system was first implemented in Peercoin, with 30 days minimum coin age. Validators have the right to forge a block based on the coin age of their stake. If the validator’s stake is idle for 30 days or more, they may be chosen to forge the next block. The bigger and more idle the validators stake it, the greater the chance of being selected (compared to PoW systems, where hashing power is the determining factor). After a stake forges a block, the coin age is reset to 0 (you have to wait 30 days to forge another block. The coin age has an upper limit of 90 days to ensure fairness.