My 95th+ percentile daughter was too tall for the Britax 65# seats at age 4. She was too tall for a Radian by age 6. I think average-to-small kids can be expected to last until about age 5-7 in a Britax 65# seat and about age 7-9 in a Radian.
She's 97th percentile now, 8 years and 8 months old, 4 feet 8 inches tall, 75 pounds (wearing girl's size 12 bottoms, girl's size 14 tops, or women's extra-small), and does
not pass the five step test in any of the vehicles I've had her sit in without her booster. I've heard from others that it may be possible for her to pass in certain vehicle third rows, but I haven't personally come across a seating position/vehicle where she passes.
It was
legal for her (and all her friends who are about a head shorter than her
) to ride without a booster ages ago, but she's the only kid I know her age (and size) still in a seat.