There's no real rules on this as far as I can tell, despite my research on the topic, including this other question on the topic of living forever by polymorph. From a practical perspective, you may as well consider this to be about biological age. The biological age is how we describe a creature: a child, a young adult, middle-aged, etc. What does the spell's rules say about biological age? Absolutely nothing.
Therefore, it's safe to assume that the creature's life span shifts along to its new form. A young half-orc becomes a young elf, an old half-orc becomes an old elf when they change form. Similarly, changing back to the original form would have the same effect, converting a middle-aged elf to a middle-aged half-orc.
In other words, there's no way to kill a creature using this spell to affect their age. There are abilities out there in the world that can affect the effects of aging, but this isn't one of them. They remain the same biological age for the purposes of this spell.