Adam went into his size 2 bamboo LLs at about 18lb (6ish months); they are not the trimmest of nappies, and are not low-rise, but they are so reliable especially when combined with a Motherease wrap (I use Airflow but have read good things about the Rikki).
This is how they fit Adam at 18lb
I also use Tots Bots Bamboozles and Stretchies, and have to say I prefer the Stretchies. For one, like cath just said, they are practically BTP, with poppers to adjust the rise. They are also a lot more flexible and really slim fitting, while being just as absorbant as the regular Bamboozles. And the velcro tabs are bigger and seem a lot sturdier than the ones on the regular Bamboozles, which are very small and are about to fail on mine (so much so that I don't actually use them any more even there is loads of life left in the actual nappies). The velcro on the LLs is pretty good.
eta: the popper fastening regular Bamboozles are also BTP, as the rise poppers down if necessary. If I were buying my stash again I think I'd have included some of these and completed avoided the velcro TBBs.
and I'd definitely avoid the microfibre LLs. they're super soft and lovely but we got leaks all the time. that said, they might be better with a decent wrap, I was still using my LL wraps back then, and they wicked round the legs and the waist. I'm sure they'd work better with a Motherease. And they do dry very very quickly, they are almost dry right out of the washing machine.