Car Seat Question

We have 2 Graco Comfort Sports now which go to 40 inches and 40 lbs as the limit.
We had purchased a Graco Platinum(?) Cargo booster for our old sitter. We'd like to replace the Comfort Sport in Mark's car (the straps are a mess) with this - it appears at 36.5 inches and 31 3/4 lbs Alan can use it with the harness.

I'd like to hold off replacing the one in my car until we can get a non-harnessed one - at what age/size/weight can Alan use something like a Graco Turbo Booster safely?

Thanks.