We had a 2004 Prius that we sold at 252,000 miles. We replaced the big battery one year before we sold it.
But any cars with that kind of high mileage start to need a LOT of repairs. Wheel bearings. Leaks in various places- like the master cylinder. Our AC went out. It still had life left, but was going to need a lot of $ put into it. We sold it for $2000 and bought a used 2010 for $10,000.
150,000 miles is pretty young for a Prius. Given the timing, I'd keep it for a while longer and use grandma's car for repair funds. Casually keep an eye out for a Gen III Prius to upgrade. Now, nearly a year after selling our old one, we're really glad we have our 2010. Big improvement.