This is just out of curiosity ... I did not want to hijack SeptemberMom's thread which made me think of this.
How strictly do you follow the manufacturer recommendations for your car seats? I saw that one reply mentioned that many car seats allow for the use of 5 point harness up to 90 lbs. That is the average weight for a 12 year old boy (11.5 year old girl) and I am sure there are many that fall below the 50th percentile. Do you really make your pre-teen or teen children wear a 5 point harness? How do they feel about it? Do they gladly comply or feel that you are too strict? (I realize, you are the parent, it's not their choice - just curious what they think about it.)
I will admit that I am probably waaaay more lax about car seats than most on this board (at least from the posts I have seen here over the years). I turned my kids forward facing at 1 years old and we will be letting DS use a seatbelt with his booster instead of a 5 pt harness when we get back to the US in June (he is 6 and is about 45 lbs). I do not know any other person IRL that still has a 6 year old in a 5 point harness (most everyone I know started switching at 4).
The car seats we have are made for harness up to 50 lbs and seat belt for over 40 lbs. I obviously would not use the seat belt for a child under 40 lbs if the car seat is not designed for that but I feel comfortable switching to the seat belt at this age and size.