K.C.
If it's not listed in the safety ratings, I probably wouldn't do it. We are very happy with our Britax Frontier. We also really like the Graco Nautilus, but the Britax is a little better. Both are five point harnessed seats that convert to both a high back and a low back booster.
My son is 5.5 years old and approximately 46" and 48 lbs. He sits in it harnessed.