My baby was 12 weeks prem. The paeditrician said go by the legal age and we started when she was just over 6 mths legal, less than 4 mnths gestational. (I could have kissed his feet it was such a relief.) I assume your baby is still monitored by the hospital? - so ask the consultant. Your health visitor will probably say wait until 6 mths gestational, but they tend to think of all babies as the average baby.
i would wait until they can do it IYSWIM. that may well be later. so i'd probably start offering when they are 6 months in true age, but perhaps expect it to take until they are nearer 6 months adjusted. no rush. they'll do it when they're ready...nothing to do with you really!
tbh BLW takes a lot of the doubting out of it, because the whole point is that you wait until they can do it. for some kids it'll be later, some earlier. there seems to be a lot of confusion abotu what to do with preemies, doesn't there? it's hard to get the reasoning out of the docs, i'm told.