The britax marathon has the capability to keep the kids in a 5 point harness from 20-65 lbs which is longer than most other carseats. I have my 2 year old in there now. And my 4 yo son is in the britax frontier which also keeps them in a 5 point harness much longer (25-80lbs), then in a booster until 100.
Both kids are very happy with the seats. My son loves the armrests that go up and down and the cupholders. The only con for the frontier is if you have captain seats (I have a toyota sienna) the seat feels like it wiggles a bit where the Marathon feels rock solid when installed. I had the police install both for my so I know it was done right but I'd feel better if I didn't feel as much wiggle with the frontier.