It doesn't matter how old you are, it matter how old you LOOK. In heavy metal, people are getting older and older. Consequently, nobody really gives a crap how you look in metal anyway. Hip hop is different.
The reason why it's hard for older cats to break into the music game is because of generational issues. Music is an area where the youth capitalize on their generational experience to put out a creative product that speaks out to their specific generation. IMO that's how cats initially get signed to a label. I remember a dude named Scatman John back in the day who was old as hell-- I think he was 45 or 50, but put out a top 10 song in the UK and Europe because it was in tune with the dance music of the 90's (rap/techno, video here:
He sold six million records. But he had to change his style from doing jazz scat in clubs, to doing scat over techno-- which appealed to the youth market at the time.
It's all about appealing to the youth generation first, at least for major artists.
But if you break it down, it really is about cultivating a fan base and gaining a following-- then marketing to that fanbase. That's all you really have to do. Now are you going to break into the industry at 50? Probably not unless you're a slept on genius. But it's possible to work extremely hard and gain a following, one that can pay to see your shows and pay for your music.
The music game's tough, age makes it even tougher.