I have two peach trees. They produce lots of peaches but they never get big enough to eat. They stay small and hard. On one tree they stay green and the other tree they turn a little yellow.
If they stay on the tree through to the fall, it may be that your tree is getting just a little bit too little chilling. Without enough chilling, peach trees will not flower properly. If the chilling time is just a little bit short, the fruit will form, but will not ripen. You may want o check the variety and make sure it is appropriate for your area in terms of chilling time.
If the fruit falls off before fall, then this is a pollination issue. These articles will help you with that: