coins that have been unspent for at least 30 days begin competing for the next block. older and larger sets of coins have a greater probability of signing the next block. however, once a stake of coins has been used to sign a block, it must start over with zero "coin age" and thus wait at least 30 more days before signing another block. also, the probability of finding the next block reaches a maximum after 90 days in order to prevent very old or very large collections of stakes from dominating the blockchain.