Damian was about 5... he had a mild scoliosis which made him fall asleep upright, but end up with his head in his lap in boosters... So when tax time came and the sale went, I was going to buy him a Britax Husky... but my grandma surprised me with a Regent (still bought the Husky, but for Jeffrey)... he outgrew it 9mo later, when he was 6.5yrs old, but there you are... he had a HUGE growth spurt, almost all of it torso height)
Jeffrey was 4.. he hadn't been in a booster long and I just found out about the seats that lasted longer. Well, after I got a Scenera for Ruthie to use as a spare seat, I realized I could put him in her Marathon... so that's what I did (at 13mo, she was already nearing the top of the shell on the Evenflo Triumph 40lb limit, so it's not like I could use the seat we'd had prior to the Marathon, which I bought with the money I got from relatives for christmas)
I would have left him in it until he outgrew it, but as I said, then I got a Regent AND a Husky, when I thought I would only get one.
Anyway, he was happy to ride in a harnessed seat... until his brother got a booster and he wanted one, too... I compromised and let him make the move at 6yrs old, 50lbs, and over 50", so I wasn't disappointed... and he probably would have stayed harnessed longer if I'd insisted, but he was more than able to ride in one maturity-wise.
Damian had gotten bruising on his back before being harnessed, no matter how padded the booster was... but in the Regent (or in the marathon... he was AT the top slots and Jeffrey was younger, but if I had just him and Ruthie, I let him use it), he was much more comfortable. Fortunately, his spine stopped curving in that 9months... and then he was able to sleep bolt upright in about any booster... which was also good because he kept growing that fast and outgrew the Old style Parkway (the tallest booster on the market at the time) at 7yrs old... hit 4'9" by 8yrs... and is now a whopping 5'6".
Ironically (and this is my point, I guess) Damian misses being harnessed... if I show him a pic of when he was in the Regent, he'll say how he loved that seat, because it was so snug and he didn't have to think about sitting still or worry about falling asleep or anything.
What Jeffrey misses (probably because he was in a booster so little early on and then was harnessed for so long that he was really ready for the booster move moreso than his brother) is a highback booster. he still thinks that vehicle seats should come with "pillows" (headwings) for sleeping. He CAN sleep bolt upright (has been able to since he could sit unassisted), but he prefers some headwings for support.