I dont know if this was already made, but how can he be five hundred years old?

Healthy diet and exercise.

Because he was cursed by a misfired spell that turned his friends to stone.

A better question would be how he can be only five hundred years old, if he was part of Ysgramor's 500 Companions?

He couldn't count past 500.

The only information I have on Ulfgar is UESP. Is he one of the 500 Companions? UESP gives no indication of that.

According to his in-game dialog, he says he was. I can't remember the exact quote, and I'm a bit lazy right now to look it up in the CS, though.

To be fair, he doesn't say he's five hundred years old. He said he was turned to stone five hundred years ago. It might be a hint that something more is going on here than meets the eye. Or, that time passes a lot more slowly for a stone.

True, but he was a Nord, who aren't known to have extended life spans. Being that he says he's lived five hundred years because of that spell, that doesn't fit with the idea that he was around for a few thousand more before that.