The advice is not to use them until their head is too big to go through the hole once the shoulder straps are done up - can't risk them wriggling down into the bag and struggling to breathe.
I've only seen 0-6mth ones, have got one so far and hoping to get some more as pressies and will be getting some blankets for when baby is first born as unless he's really big he'll be too small for the ones I have.
Oh and tog rating depends in how warm your house is. Mine is due end October and I'll be keeping the house about 18-20C so will go for 2.5tog, by the time he's into the 6-12mth size it'll probably be summer so 1tog (+ blankets if we have any summer like this one!)
Edit - found the table for temp:
Room temperature and bedding guide
Room temperature Amount of bedding
24 degrees C / 75 degrees F or more Sheet only
21 degrees C / 70 degrees F Sheet plus one blanket, or a 1 tog sleeping bag
18 degrees C / 65 degrees F Sheet plus two layers of blanket, or a 2.5 tog sleeping bag
16 degrees C / 60 degrees F Sheet plus three layers of blanket, or a 2.5 tog sleeping bag plus a blanket