TCU,
The crew that gave you an infant belt for take off and landing shouldn't have done.
I actually looked into this, as I wanted to be able to give an informed answer to parents, such as yourself, that asked the reasons for the cut off age of two. I was told that, at two, a childs spine is starting to stiffen and is nowhere near as flexible as it was when the child was an infant. Because of this, the pressures of an adult leaning over the child in an emergency could cause spinal damage to the child.
Obviously, I'm not a doctor, so can only go by what I was told by this person, who is in the medical profession. I have to say, it does seem to be logical, but if anyone knows any better, please feel free to correct me.
Jsl