My son is 5, about 45 - 45.5" and using the top slots of the Frontier85. About 1.5" to grow the way that it is currently installed but typically has about .5" (not sure what's different this time, more upright or something? Not purposely, just the way it happened to be when I finally got it tight on the 8th attempt, but I'm trying hard not to uninstall). He's at the same spot, shoulder-wise, as when we got the seat close to a year ago, just growing legs this year (finally!)
He hit the top slots on the Radian at 3yrs.... older three, not sure exactly how many months anymore, but a couple months before he turned 4. He was 41" tall on his 4th birthday.