The bit in OOTP implies (though it doesn't precisely say) that the order is from oldest to youngest:
(I've not got onto book 6 re-read yet, so if there's anything in there I may well have forgotten it.)
Is this vouched for elsewhere/by JKR? Is there any reason why two of them couldn't be twins (just with the elder twin marked first) - or, hell, why they couldn't be triplets (apart from sheer improbability, obviously, which is beside the point!)?
If there's a definite age gap, does it say anywhere what that age gap IS?
(Yes, I have fanfic writer's angst here, does it show?!?)