Hi and welcome
Your concerns honour you. While there are a few guinea pigs that cannot be bonded because they have lost the ability to identify themselves as a guinea pig, the majority actually is never too old for company. I know of one 6 year old single boy who finally found companionship and a loving home with another old boar via a rescue.
My 2 year old Dylan is another example; he is another single boar being kept in a tiny hamster cage and ending up with atrophied back muscles as a result. He now lives with a a very happy life with a baby companion (he had the choice between two that were born here during emergency holiday boarding a highly shop pregnant sow and her companion for a local lady) next to another pair of boars.
Age is less a concern with guinea pigs than character compatibility and mutual liking. Even next door company with interaction through the bars can do the trick for those piggies that have never had the benefit receiving the social guidance during their formative months and who are too aggressive/over the top as a result (usually because they are afraid and insecure). Sadly it is exactly this point - character compatibility where shops and breeders fall dreadfully short and why your shop is finding itself inundated with fallen-out boars once they hit the hormonal teenage months between 4-14 months and stop being cute babies. Too small cages usually don't help.
The key point is dating - letting your boy choose who he gets on with. If you can find a good piggy savvy rescue with manadatory quarantine/vet care that offers dating at the rescue under expert supervision, that would be best for somebody with no experience in bonding and behaviours. Otherwise trying him with a submissive single boy as they come back to the shop in would be the best way forward.
Here is the video from when dated my Dylan with two baby boys. I chose the one who gave him kisses and wanted to play with him and not the one who was trying to mount and dominate him from the word go (the other boy is now bonded with another bereaved adult boar via rescue dating, by the way).
Please take the time to read our new owners guide collection. You will find it very helpful with guides for settling in and understanding guinea pig behaviour.
It also contains a link to a list of recommended US and Canadian rescues, as I assume that you are from one of those countries with your use of 'pet store' instead of UK/Australian 'pet shop'.
Getting Started - New Owners' Most Helpful Guides
We would be grateful if you please added your country and state/province to your account details (via your username on the top bar), as that allows us to always tailor any advice to what is available and relevant for you straight away instead of having to keep our recommendations as general as possible to fit any members from all over the world.