FF yes, b/c it's about bone density and development, probably around 6yo.
Guess, a better question would be would you still 5pt harness at more like 10yo?
I was kind of discussing exactly this w/ a friend w/ a Dwarf son. She FF him at a year old and I wasn't impressed. Told her as long as my 5yo was RF, her 1yo w/ a larger than normal head belonged RF for the same reasons and then some...She agreed to turn him RF until I turned ODS FF, and did so. He's now 2 1/2 and IMO should still be RF, as is my YDS but...At any rate, at some point in the next few years, she'll have to decide what/how to deal w/ 5pt vs booster (40#min here). I'm hoping he doesn't make 40# any time soon b/c I hope she'll follow the law on this one...
tam