although they recommend waiting til 6 months, some babies show signs of needing to be given more than milk earlier. if you introduce solids before six months you must avoid gluten and certain other things to reduce risk of allergies and so on
also, i notice your LO was a preemie...the reason they say to start at 6 months is because thats when the iron levels that you passed over in pregnancy start to disappear so baby needs to take in its own iron to replenish its stocks, but with a preemie it wasn't in you as long as nature intended so its iron levels are likely to deplenish before six months so they are usually recommended to be weaned earlier than a full term baby (my friends preemie was 7 weeks early and she was advised to wean at 4.5/5 months for the above reason)