The Guardian is not a mage. We don't know the Ashes heal because of Lyrium. The Dwarves resistance to magic is an evolutionary adaptation to protect them from high quantities of lyrium. Not the kind of thing that's neccessary if it gives you immortality.
Neither is Eamon, yet they clearly effect him.
Templars aren't mages, yet they use lyrium regularly to get their "holy" powers.
And the guardian isn't immortal, he can be killed by the Warden who defiles the ashes. As can Avernus who extends his life using blood magic. Lyrium is used regularly by dwarves for enchantments, and they don't have a single mage among them.
It's quite possible no one has experimented with the possibility before because lyrium, by its very nature, is very dangerous. Mages can't go near unprocessed lyrium without suffering deformations and death, humans and elves can't even handle it until its been processed by the dwarves and sold to the surface.
Avernus experimented with blood magic, actively looking for a way to hold off the taint, extend his life, all the while fighting a near constant demonic invasion. The dwarves don't have mages, and thus their capacity to experiment with lyrium is reduced, and the dangerous quality of lyrium makes any experimentation on the surface also dangerous.
Lyrium has a wide array of utility, and it's highly unlikely anyone has discovered everything it can do. I don't doubt that it can be used to delay old age like Avernus does with blood magic, but I doubt that anyone has taken the time to research the possibility of it because lyrium is so dangerous to work with.
I think it's quite likely that all the lyrium in the mountain created a series of side effects on the ashes and the gauntlet itself, and the guardian's longevitiy may simply be part of those effects.
The fact that no one has discovered how to do it does not take away the possibility of its, well, possibility.