Doesn't make it a wise idea. Your still pushing the envelope whether Toyota or any other brand.16 years on a modern Toyota P-OAT coolant is not that much, there's plenty of Toyota's at 16 years and 180K miles with the original coolant that are fine. Unless you see sediment, or any other problems with the old coolant I would just refill with new P-OAT coolant and call it a day.