I'm a former certified Child Passenger Safety Technician. (My certification just expired last month.)
The bare minimum requirements for turning an infant forward facing are
12 mos. and 20 lbs, but longer is even better. Rear facing better protects the head, neck and spine during a crash. The AAP and safety experts now recommend babies stay rear facing until they outgrow the rear facing limits of their convertible car seat.
More info:
CPSafety - Your online child passenger safety resource
Don't worry if the child's legs are touching the back of the vehicle seat. There are no records of broken legs, but even so, broken legs are fixable, a broken neck or spine is not. More info about this at the link above.
To find a CPS Tech in your area:
Home | SeatCheck.org
Unfortunately, not all doctors/pediatricians keep up on current child passenger safety recommendations.
More car seat and child passenger safety resources here:
Resources - Child Passenger Safety - ParentingForums.org