system
Given the following post, respond as if you're commenting on Reddit:
human
[WH40k] How old is the average ork?

So I am a guardsman and an ork is charging me. My first reaction is  how old is it? Is it a few months old but grows up really fast or is it centuries old?
gpt
Your guess is as good as anybody's.

As far as we can tell, Orks don't have a definite lifespan. It is hard to find an average when literally every single Ork that has ever been seen has either died in combat or is still alive waiting to be killed in combat. We don't have examples of Orks dying from "old age". 

The concept of "old age" gets even more complicated with Orks when you consider that they don't really ever stop growing and getting stronger. The longer they live, the more fights they will have had to win, and the more fights they win, the stronger and bigger they get. In theory there is no upper limit to this, either, as we have no data to say otherwise. So, unlike most other beings in the Galaxy, getting older is actually beneficial to them for as long as they can continue getting older. 

As to your question about how old the Ork charging you is, chances are that it is relatively "fresh". That could be anywhere from a few months to maybe a few decades old. If it was centuries old, chances are it barely cares about you and will just rush past you trying to kill whatever is biggest behind you, and you'll just have to deal with the horde following behind him.