I don't think the author means to justify age discrimination so much as to use it to underscore a point, which is that programming knowledge is something that takes (on the order of) 5 years to acquire rather than 40 years to acquire. Whether this is true, I am not sure. I don't really have enough experience to justify making any claims like that.
I think what he's really saying, to make an analogy, is that a programmer is more like a black smith than like an alchemist. A blacksmith is concerned with his tools and his technique, an alchemist is on a search for truth (or permanence or glory). Now he's also saying that being a blacksmith sort of sucks: your tools get old, you are smelly, it probably hurts more to pound your anvil when you're 50 than when you're 22, etc. Whether you believe that depends on your taste, whether you care about creating a good product or embarking on a potentially (very likely) fruitless search for truth.
I think what he's really saying, to make an analogy, is that a programmer is more like a black smith than like an alchemist. A blacksmith is concerned with his tools and his technique, an alchemist is on a search for truth (or permanence or glory). Now he's also saying that being a blacksmith sort of sucks: your tools get old, you are smelly, it probably hurts more to pound your anvil when you're 50 than when you're 22, etc. Whether you believe that depends on your taste, whether you care about creating a good product or embarking on a potentially (very likely) fruitless search for truth.
It's pretty obvious what hackernews prefers.