At what age does true grumpiness start?

... On the upside, I did read that this is normal. You get really fucked off in your forties, then gradually cheer up...

Until you have a stroke, heart attack or brain tumor and die.

Sounds about right. My dad progressively got more and more grumpy in his forties, then had two more kids which made it worse and it peaked last year when his blood pressure got to the point of about-to-have-a-stroke.
His doctor gave him meds etc but the funniest thing was that he was told to keep calm and relaxed during this period, which looked painfully difficult. My two younger siblings had to practically be kept away from him in order to not piss him off. He's pretty chilled out these days now.

So yes, I think we as males are destined to become grumpy and short-fused as we get older. I think people tend to mellow a great deal in their fifties though, as I've seen with others.