Any age is fine for giving a guinea pig a friend and I'm sure he will thank you for it. Read this page https://www.cavyspirit.com/sociallife.htm for more information on how to go about it.
If you have an older pig, a young pig (same sex or altered) usually works well, or perhaps another laid back older pig (2 yrs.+). Most grieving pigs will warm up to a new buddy.