For my kids:
main car - 4yo and 30lbs OR seat outgrown
Secondary cars - 3.5yo and 30lbs OR seat outgrown
Occasional Rides/Other rare situations - 3yo and 25lbs
Planes - 2yo and seat minimum weight/height (2.75 for DS b/c he wasn't 34" until then)
I would turn earlier for a genuine medical need that could not be resolved any other way. I will not turn for complaining
For other people's kids:
In my car - 3yo and 30lbs (using one of my seats or their own) OR seat outgrown
In their car - 2yo and 20lbs
I won't say anything to people once their child is two unless they ask about my kids. If their child is under 2, I will gently suggest or move the conversation onto the subject, but only once. If I've discussed it once and they still have their 1yo FFing, I won't bring it up again. If their child is under 1, I will mention the illegality a second time, but then I drop it. If they bring it up, then I'll discuss, but I won't badger people.
I've gotten much more positive responses by letting people see what I do than by constantly nagging them. I've kept several kids in my church group out of expired seats, rfing until 2, and harnessed until 4-5 just by being open, discussing, and encouraging people. I've even got an 8yo still in a booster b/c mom looked up the five step test after talking carseats with me and realized that while her child had turned 8 she still needed a booster.