They're also supposed to be in them until they're 10 or so, these days. You're likely to go through 3-4 seats per kid—infant (often also a carrier), toddler, then either a larger car seat and later a booster or a convertible booster that can do both.
And yeah, you absolutely hand them down to younger kids. Throwing out old car seats is nuts and not something people actually do unless 1) they have more money than you know what to do with, or 2) the seat looks like it's in bad shape or reveals on inspection that it's damaged or deteriorating, or 3) you just don't know anyone who could use it.
>Throwing out old car seats is nuts and not something people actually do
Yes they do. Target alone is a counterexample over 500,000 strong.
Fear is a powerful motivator. My wife and I are constantly lamenting the trading on Americans’ obsession with child safety, but even we found this article allaying some lingering fear that there might be some unknown reason to stop using the seat we picked up off the side of the road.
The counterexample needs a huge caveat. Outside of one-off registry discounts and occasional manufacturer end-of year/model/design sales, the Target trade-in deal is one of the best deals you can get on baby/child gear. Parents in my local groups will hold on to seats from accidents and more, just to get or give away these 20% coupons.
I suspect most second hand car seats are immediately traded in. If you watch the market most second hard car seats are sold at the time of the target sale.
Even adults would be safer in a car seat like the ones kids sit in, if anyone would be willing to use one.
If you scale up a toddler car seat, it looks a lot like the safety seats in race cars or fighter jets: the seat wraps around the outside edge of the body and upper legs; the head rest wraps around the sides of the head with padding; the restraint straps have a 5-point attachment system.
And yeah, you absolutely hand them down to younger kids. Throwing out old car seats is nuts and not something people actually do unless 1) they have more money than you know what to do with, or 2) the seat looks like it's in bad shape or reveals on inspection that it's damaged or deteriorating, or 3) you just don't know anyone who could use it.