How soon can a premature baby fly on plane?

Katelyn • | Blessed With My Twin Boys July 11th, 2018👨‍👩‍👦‍👦 | July 21 Is My Glow Day🎂| 3 Years Long & Hard🔐|

My twins were born July 11th at 27w4d, they're still in NICU and currently now 1 month old. When they're released which could be in September or first week of October, can they go ahead and fly?