Trailing wife is right.... Airlines have very strict policies on these things because they are responsible for your baby's safety. BA have baby seats that are suitable for older babies (kinda like a car seat in that they are in a comfy seat tilted back as opposed to a bassinet). Not sure about other airlines but policies do differ between companies.
We recently arrived from London (on Cathay) with our quite sizeable 7 month old and bought one of these for the journey:
http://babyproducts.about.com/od/travel/gr/CARES_harness.htm
He was boarderline for the min weight...It worked great. It does require them to have their own seat (no, we weren't paying - it was relocation)
Some airlines allow you to take on your car seat...but it does depend on the flight class and airline policy...again, check with the specific airline.
The good news is, whilst its not always possible to get a bulkhead seat with a spot to put your baby, my experience on a few different airlines (we've ended up having a lot of unexpected travel this year!) is they really do try and accommodate the request. Still..bottom line is they may already be reserved.
Without going completely off-topic, some airlines (qantas!) won't even allow you to pop your baby on a rug on the floor for a bit to stretch out on a 12hour flight. But on the connecting leg a different flight crew encourage it. The rules drive me a bit crazy but the inconsistency is maddening!
Hope it goes smoothly and the crew are accommodating
