Hi and welcome!
Thank you for asking and doing your research before you get guinea pigs (plural).
Guinea pigs are group animals; just because there is just one left doesn't mean that it is not dreadfully missing it companions and that it is happy to be on its own - rest assured that it is desperate for company! You should never keep a single guinea pig; however much you love it, you are willingly and deliberately failing a basic need of it.

They should not be kept single and they are never too old for company. Sadly Switzerland is still the only country where it is illegal to keep and sell any group animals as singles.
There are only very few guinea pigs that are not interested in their own kind. The vast majority of single guinea pigs are cavies that have been failed often more than once by humans.
Companionship
Guinea Pig Facts - A Short Overview
This guides are part of our information pack for wannabe owners:
Are Guinea Pigs For Me? - A Guide