Samy27

Till what age does an average child use the 65lbs or 50lbs convertible carseat?

Skyburning

I was 7 when I weighed 50 lbs but I don't know if thats average.

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Trina

A traditional car seat harnesses up to 40 lbs, which will generally last an average sized child to about 4. A car seat that harnesses past 40 lbs. and has higher harness slots will last a few years longer. You really can't go strictly by age because kids vary so much in size. My DD (10) is just over 50 lbs. but she's too tall for a Britax Marathon, even though it goes up to 65 lbs.. My niece (currently 7) is about 46 lbs., but is long torsoed. She outgrew her Marathon at 5 because her shoulders were above the top harness slots.

If you have a toddler who is big for his/her age it's a good idea to buy a larger car seat that harnesses past 40 lbs.. CPS Techs recommend kids stay in harnessed car seats until at least 4 yrs. and 40 lbs.. Longer is even better.
CPSafety - Your online child passenger safety resource