What is the appropriate age to allow a toddler to sit "forward facing" in a carseat? And why?

No < 2yo & to wt lmt. Any kid facing forward is at risk of spinal cord damage when the unrestrained head snaps forward in a crash. See www. Joelsjourney for the effect on a 33lb 15mo. The aap was 10+ yr behind the nhtsb in recs to rear face till 2 & that is a travesty. As neck strength improves risk declines. I have cared for a quadraplegic infant from this injury. Parents or drs who ff @ 1yr are uninformed or negligent.
1 year. You can do it at 1 year of age, but nowadays the recommendations to keep him rear facing as long as you can.