How old does a Pawpaw tree (Asimina triloba) have to be before it produces fruit?

I planted 6 Pawpaw tree seedlings in my yard in the coastal area in the PNW 10 years ago. These trees seem to be very happy in the area where I planted then which gets partial sun under a canopy of Red Alders. The hight of the Pawpaw trees is now 8 to 10 feet tall with beautiful green leaves. Each year I watch for blossoms but another Spring time has come and gone and still I see only vegetative growth. Maybe there is something that I can do that would stimulate the trees to start producing?