My DD is in the Marathon that was out before the 70s were produced. She'll be RF til 35lbs and FF to 65 or until that seat expires. Then we'll figure it out. She's never been FF except in a taxi on vacation and she does just fine figuring out her legs. Google and you can find 4 and 5 yr olds RF in seats. They dangle their legs or sit cross legged. It's really not as big a deal as you might think.
DD is 3. She climbs right into her seat and understands that she's little so she has a special seat and doesn't ask to be forward or in just a seatbelt. In fact, her toys often need to be belted in, too, but that depends on human passengers.
I think that the more people read about things like internal decapitation and manufacturers give us the seats we need, the more older children will be RF longer and/or in a 5 pt harness longer. I rather feel that we are in the middle of accepting the new idea/standards but everything will catch up in the end.
Somewhere around 12-18 mo. we moved DD to the first Marathon and then got the second when we were able to purchase. Keep an eye online for sales. Most include shipping. We are very happy with our Marathons. She was in a Keyfit 30 prior (which we also liked).