Why not from birth?
Veganism is not a fad- and will eventually become the norm. This is as sure as plant-based egg substitutes are plant-based egg substitutes.
Forcing a child to eat meat when she doesn't want to is child abuse. You might get away with it now but in ten years time it will be as much of a crime as smacking.
Besides, veganism is the ethical choice. Your child has high standards; Why would you drag her down?
We have a friend whose six-year old daughter is a conscientious vegan. It's not a problem for either of them. Vegan food is everywhere. Sign of the times: Tesco's cafes are not only offering but advertising a vegan breakfast.
On a point of information I've got as far as giving up meat- but I stick at fish and dairy. I'm not proud to be a slave of my conditioning- and I'm happy the younger generation are more ethical than I am.