my doctor said to keep the baby in the infant seat as long as possible. The rear-facing infant seat is the safest option for kids until they don't fit anymore, basically - if their legs have to be scrunched up to sit in the seat, or if their head comes up higher than the back of the seat... OR if they exceed the weight limit on the seat... then it is time to switch them over.
My daughter switched over at about age 1 (she was 17 lbs) - she was getting too tall to fit in the seat.
Switch over when baby doesn't fit, or ask your pediatrician. They usually stay updated on that sort of safety information. Keep in mind that stores want you to by the next seat WAY before it is necessary or even recommended, and if you followed their advice you would be switching every few months!
Once your baby is in the convertible car seat- follow the same rule of thumb- as long as they fit! It will have similar recommendations for weight an height somewhere on it if you are trying to decide on which to get!
-M.
wait- actually I was wrong... I switched her over at 20lbs... the day the she weighed in at that- we BOUGHT the seat when she was 17 lbs.... so technically she was a few months older than one LOL- mommy brain!