There are basically two types of state laws regarding RFing:
1) Essentially requires all car seats to be "properly" used.
2) Requires children under 1 year and/or 20 lbs to be RF (that is, kids have to meet both age and weight requirements to FF).
All states currently have one or the other of these laws in place. Some states have both.
There is no federal law that requires parents to keep their kids RF to age 1 & 20 but there is a federal agency which requires car seats to be labeled in a certain way in order to be approved for sale in the US. That agency requires forward facing car seats to have 1 & 20 as bare minimum requirements (manufacturers are allowed to put higher limits, such as age 2, or 22 lbs or 34" tall to name a few).
That said, there are like two car seats on the market which somehow fly under the radar and don't specifically say children must be 1 year old to use them FF. They're not in compliance with the labeling guidelines but apparently nobody really enforces it. They still both require a child to be 20 lbs before riding FF. I would still argue that "proper use" would be at least 1 year old for FF because those are NHTSA's guidelines whether the manufacturer specifically says so on the label or not.
So, basically, with a couple of very rare exceptions in some states with certain car seats (maybe), anybody who forward faces their child before a year or before 20 lbs is either in violation of the "proper use of a federally approved child safety seat" law or in violation of a specific RFing law.