Looks like she fits on the back pretty well really!
I used to ride off road a lot when in my late teens/early 20's and my mates kids were always along for a ride, they used to love it. My son's been on the back of my bike when he was smaller, just up and down the road a bit slowly but now he's graduated from a scooter to a CR 125 and loving it. (also my mates son who is on an SV 650 now) Nothing wrong with starting them young in the right environment, my wife used to go on the back of her Grandad's R100 when she was little round London.
When should you start on bike then? I had a TS 100 when I was 12...... off road of course! (mostly)