Thank you for your responses. I was thinking age 8 since the seat will expire by then 5 or 6 years is the time frame they are good for.
Not many seats have that short of an expiry. The Frontier is 9
years. That's why it's able to go from 2 to beyond 8 years - Britax was targeting this seat for kids coming out of rf taking them through to 4'9". That's also why the age minimum is 2, they're trying to model rf to age 2.