My MIL weaned DH from breast to bottle when he was 18 months old and is after me to do the same with our DD (so she can go to sleep without me there to nurse her down, mostly, I think). I am in no way planning to do this. I'm planning to wean gradually over the course of the next 6 months as DD gets better about eating solid foods etc. I told my MIL that "they" don't recommend starting bottles after about age 6 months. However, when she asked me why I had no good answer.
Now, there are two reasons I can think of for not introducing the bottle at this stage -- bottle feeding to sleep is bad for the child's teeth, and it would be hard to get her to take it, besides which she's well able to drink from a cup, grown-up sippy bottle, etc. However, the argument that it would be difficult (and pointless) to get her to take a bottle isn't the kind of thing thing that would carry much weight with my MIL.
So, can any of you think of any other reasons not to introduce a bottle at this age? I'm just curious, because normally I can come up with lots of reasons for things like this, and this one has me stumped.