
Laws can vary by state and municipality. But putting laws aside the safety of your child depends on being in a property fitting and installed child safety seat.
Only rear facing car seats should be used for infants. Rear facing seats should be used exclusively until the child reaches one year of age and 20 pounds. Upon one year and 20 pounds a child can be placed in a front facing seat.
Motor vehicle crashes are the #1 killer of children from ages 1 to 14. About 50% of these deaths to children under 5 involved children that were unrestrained. Of those that were restrained, misuse is reported in 80-95% of cases. Injuries requiring hospitalization are even more common, and many involve the head, neck, and spine. Some of these injuries are permanent.
A five point harness system is considered to be the safest. Also look for wide, twist free straps. Two piece chest clips are desirable because they reduce strap twists and make it more difficult for the child to detach. Front harness adjustments are very important. The easier the tightness is to adjust, the more likely it is that you will adjust it properly every time. A tight harness can increase ride-down time, reduce the forward movement of the head in a crash and reduce the overall risk of injury.
Many manufacturers now put "expiration" dates on their carseats. Six (6) years is the general recommendation. At most, 10 years is the accepted maximum lifetime of a carseat. The reasons for these limits involve possible degradation of the plastic shell and other parts, the possible loss/breakage of parts, and the fact that older seats will often not meet current government safety standards.
The center of the rear seat is usually safest since it is farthest from a possible side impact, but only if your carseat fits well in that position. Any position in the rear seat is acceptable unless prohibited by the vehicle or child seat owner's manual. The seat behind the passenger may be slightly safer than the seat behind the driver, since it allows you to unload the child on the curb side, allows you to see your child more easily from the driver's seat and is very slightly less likely to be hit on that side in a side impact.