Have your guinea pig checked for cataracts. They are not all that rare in older guinea pigs (whereas glaucoma is much, much rarer!) - similar to human old age cataracts, only that they are not treatable in guinea pigs.
Congenital cataracts (i.e. genetically inherited ones) turn up in younger piggies, sometimes as early as birth, but more common in the second year of life. it is important that while there is nothing your vet can do, do make sure that there is nothing else going on, like an infection.
Whereas glaucoma is caused by too high pressure inside the eye, in cataracts it is the lens going opaque.
Guinea Lynx :: Eyes
Thankfully, eyesight is not the strongest sense in guinea pigs and they can still lead a perfectly normal, enriched life. As their eyesight wanes, their other senses will adapt to cover for it.
You may find this thread here about a guinea pig with sudden onset congenital cataracts comforting:
Turning a blind eye on mischief - another piggy story