In the EU, carseat/booster seat rules apply to kids under age 12 – based on height… 135cm (53.1″)
Basing it on the kids size makes sense to me. My kids are giants – my 9 year old is over 5′, and my 7 year old is 4 and a half feet. My 7 year old is quite a bit taller than some of his older brother’s peers.
I like the EU system of basing it on height much better than the random laws around the US. In Pennsylvania kids need to be in boosters until age 8. My almost 8 year old is 4’6.5… taller than many 10 or 11 year olds. We have to haul a booster with us when we visit family in Philly.
I also want to agree with flu about hand me down seats… The plastic gets brittle over time in a hot car – most manufacturers suggest retiring a seat after 5 years… but you see carseats on craigslist, at garage sales, etc that are much older.
