It is not a problem to harvest or pick onions early. They can be picked and eaten at any stage of growth. The most mature onions should be picked at the end of summer. These can be easily stored for the fall and winter months.
If you want to pick the most mature onions, you will be able to tell when the onions have stopped growing. In fully mature onions, leaves tend to lose color and the top of the bulb tends to bend over. This should not be of concern as this is the natural course of growth for the plant. The onion will not grow any further at this point.
It is advised not to leave the onions in the ground for longer than 2 weeks after the tops die as they may be exposed to rot. Picking onions on a hot and dry day and allowing them to dry in the sun will also help preserve them.