Why, you ask? The law (and most seats' requirement) is 1 & 20 lbs. the AAP recommends at least to turning 2. Well, the NHTSA recommends 4. On the one hand, I know they're safe in a well installed & top tethered seat at this age, and they have ridden forward facing in my mom's car and on vacation.
But, we spend A LOT of time on the road, have small cars (namely my Corollla) that can't compete with the monsters on the road beside us, and really, why be less than optimally safe with your kids?
Now, my kids will likely fit, given their average height and below average weight, rear facing in their seats to 4.5 or even 5.5 depending on how things go. Will I force a kindergarten kid to rear face? Of course not. But my preschoolers are happy, and have never complained (even after riding in forward facing seats), so why not keep them as safe as possible?
A couple of links to read: