Here's my understanding of it - now I could be wrong - I'm no expert, and I'm really just learning myself - so please feel free to correct me if I am wrong.
Babies's stores of iron start to run out around 6 months and so will no longer get all they need iron wise from milk so this is why weaning is recommended to start then. However with premature babies because they haven't spent the last part of their time in the womb when iron stores are built up, they may need to be weaned at a different time.
Babies shouldn't be weaned too early however as the stomach hasn't developed properly until around 4 months which is why weaning isn't generally recommended before 4 months, so it's important to remember that premature babies stomachs don't develop any faster - they will develop at same rate as if they were still in the womb.
Now, I know that 'they' say you shouldn't hold your baby back from the weaning process and should go with their actual age i.e when they were born. I think it's important to consider their corrected age too.
For example my baby was 10 weeks early. Now, if I'd weaned him at 17 weeks actual - he would have been 7 corrected - now, is his stomach going to be ready for this? Is it going to develop super-fast because he was premature? I don't think so. Also, is he going to be able to hold his head up? No.
However, if I wait until he is 26 weeks actual - he would be 16 weeks corrected. His stomach will be more able to cope with it, and he will be around the age when his iron levels will be naturally starting to drop. His head control will be better too. There wouldn't be any reason why I would need to do it earlier.
This is something I've been researching for weeks now, and have read everything on the Bliss website about weaning, I've spoken to HV, neonatal nurses, gps, pediatricians at local hospital, and pediatricians specialising in premature babies at Sick Kids about this. I don't think there is enough information out there about weaning premature babies.
I have a friend whose baby was 5 weeks premauture and at 15 weeks actual - 10 corrected she gave it an orange to suck on. Citrus fruits aren't recommended until much later, but she asked hv about weaning and was told 'don't hold her back'. So she just assumed 'well I can wean her now as she's nearly 4 months' without looking into the finer details.
If I had a baby that was born 5 weeks premature. I would wean them at 26 weeks actual - 21 corrected.
Sorry for rambling - I think this was more to clear my own confused head!