To be clear, you can't reverse aging just by heating up or overfeeding an old person. The point is that aging and death on human time scales are not an inevitable consequence of the laws of thermodynamics. Entropic changes can be reversed using an external source of energy (e.g. you can use energy to set up bowling pins after knocking them down). With the right technology, it is absolutely possible to repair aging-associated damage and restore an old person to a youthful state. It's a very difficult engineering problem, but there's no theoretical reason it can't be done.