Age is going to get us all at some point and there is nothing wrong with that. I still give these older guys credit for getting up on stage and giving it a go, but for most they just aren't good as they used to be. Lets face it being in a band is a job, and in some countries it's probably a job that doesn't have a great retirement plan, (unless your Marcel from Destruction!), so getting up there every night is really no different to anyone else slugging it out day in day out until they are 70.
There are guys like Testament and Flotz who do seem to be getting better, or at least finding a very solid groove, but most bands of the era, Metallica, Slayer, Megadeth, Anthrax, Nuclear Assault, Sacred Reich, Vio-lence, Exhorder they can still do gigs but none of them are as good as they used to be. But then very few of us in our own jobs are getting better at what we do either. I know myself I've got to do shit like making solar systems and rebuilding engines because I'm tired and sick of the work that actually pays the bills. I should go into wine and beer making!