I agree with maxing out the limits of your seat, for the most part. But, if we are talking about a 18 month old who outgrew a rf'ing seat - I'd buy a new seat. As far as when I would FF - well, not counting my first two children whom are 9 and 7 and were switched too soon before I truly knew better - my answer is 2.5 years old. BUT, if I had a baby or young toddler right now, I would change that to 3. Probably not making sense... but there are now higher weight limit car seats for rf'ing, like the Radian XTSL, that wasn't around 2 years ago. As it is, my 39 lb 3.5 yr old is FF, and as much as I know he is safer RF'ing, we can't swing it right now (mainly b/c I couldn't get a decent instal with the radian in my Subaru - so I didn't have any other options).
So, yeah - I wanna say 3 years, 4 if you have a average or smaller child and can afford a seat that accomodates. 2.5 year would be the absolute minimum if there were no other options.