We waited until Charlie was 6 months corrected ( nearly 9 months old in 'real' time ) before starting him and he never looked back. Told by the paed and the maternal child health nurse to wait a bit before, as their guts are still quite immature and more in line with their corrected age.
Good luck!
( P.S. Charlie HATED the rice cereal but absolutely enjoyed pear, apple, sweet potato or pumpkin, just one or two teaspoons to start once a day and one at a time for 3-4 days. )