I have had a couple of sugar apple seedlings bloom at two years old and roughly a 1 ft. tall. I think it is because I left them in small containers for those two years. Once they got rootbound, they felt a need to bloom and reproduce due to the stress of being rootbound. Other seedlings from that same batch had previously been transplanted into larger containers with plenty of space for root growth; they are three times as tall but have never shown any signs of blooming. I have never heard of one blooming at one year, though.