DS is 7 1/2 years and about 55lbs. He is still in a Britax Regent in my car (harnessed seat) and a Graco Nautilus (harnessed) in dh's car. I just ordered a Britax Frontier 85 last night for him. We will switch him to that in DH's car (ds is outgrowing the harness in the Nautilus but is not quite ready for a booster yet). Next spring, when my Regent expires, we will move the Frontier to my car and the Nautilus back to dh's car (in booster mode). That should get ds to about 8 1/2 years before he starts using a booster on any sort of regular basis (he does have a booster seat he uses maybe 2 times a month in dh's parents car).
How old was your child when you made the transition from car seat to high back booster? - Page 3
Ds was 5 yrs and 43 pounds and started riding in one part time. He's still in the Nautilus harnessed for daily rides to school. We have a high back booster in the family car that's used for outings and errands on the weekend. I actually prefer him in the harness because he can buckle himself up properly. He still has a hard time keeping the strap untwisted and under the arm rests on his booster, especially with winter gear on.