I absolutely hate when somebody says: "I've been doing this for thirty years!" because it means, every time (without fail - and I mean it) that they don't know a gd thing about what they're doing. They're just trying to throw (what they consider to be) their weight around, thinking that all that time counts for something (it doesn't). Maybe thirty years ago, they knew everything, but then they let themselves become stuck in their old ways and refused to adapt and evolve.
That's not
necessarily the case, and sometimes it's worth listening - but in doing so, applying perhaps more rigorous standards to them. Sometimes people will use that argument when someone simply
won't listen (though if they won't listen, such comments will probably be futile anyway) but that said, I have also seen plenty of people pull that stunt: they've been doing it longer, they have some sort of higher ranking, they're associated with some random group or other, and in their mind, it makes their opinion more worthwhile regardless of how informed and considered it may or may not be.
But I've seen the converse, too: "you know nothing, oldie, you're out of touch!", often said by someone about to hideously embarrass themselves.