I always thought it had to do in part with the child's size and ratio of head to body. The dangers with the relatively heavy head whipping forward in a crash are significant. But I think people really push it too early. It's really not important that the child "enjoy the ride" - kids would enjoy it more if they didn't wear seat belts too.
This may be helpful because it explains the mechanics of a baby's neck ligaments in the event of a crash: http://www.carseat.org/Resources/FAQs.htm
This is from the American Academy of Pediatrics, which says 2 or older:
http://www.parenting.com/article/car-seats-safety.
I think you should consult with your pediatrician, who would have a better handle on your child's growth/weight and head-to-body ratio. In many towns, the firefighters (who are the first responders in the event of a car crash and whose paramedics are transporting injured children) also do car seat checks and help with installation as well as advising you about making changes. You could call the non-emergency number and ask if they do this.