Not strictly true. Whilst most older boar and baby boar pairings are accepted due to the baby being too young to challenge the older one and more willing to accept their position, it is not 100% guaranteed. Personality is the most important and a baby will still need to go through the teenage hormones which can cause tension. If you have a rescue near you they may offer boar dating for finding a good match.
I would be wary about considering neutering at 5 years as the operation is riskier as they get older.
Hi and welcome
it is great that you are considering companionship!
We have certainly seen members' boars reject one baby but be totally happy with another one, but have also seen boar pairings of any age. No piggy is ever too old to choose a friend, but it is ideally their own choice whether that is a similar aged boar or a youngster.
If you can get to one, please consider dating your boy at a good standard guinea pig rescue with mandatory quarantine/vet care that offers a meet&greet or dating service under expert supervision. this means that your boy can have a say and you come home with a companion only if acceptance has happened.
If you choose a companion, then you have to plan ahead in case they do not vibe. Please do NOT get two babies - boar trios and quartets are the constellations that fail most often, the more so the more sub-adults are in the mix.
There are a few US rescues that have a sow spaying policy but there are none in the UK and Australia. With cross gender pairings one side needs to be de-sexed as guinea pigs never stop procreating; only the older they get with an increasingly high rate of miscarriages or death of babies and mothers.
You may find these two guides here helpful. The singles guide has a chapter on companionship, the particular challenges with singles and how best to go about it. The boar guide may also help you with making a decision as it takes a closer look at boar companionship.
Both guides also contain links to recommended and vetted good standard rescues in several countries:
Single Guinea Pigs - Challenges and Responsibilities
A Comprehensive Guide to Guinea Pig Boars
Here is our illustrated bonding guide which covers the whole process and also contains chapters on aspects that are particular to boar bonding and baby bonding:
Bonding and Interaction: Illustrated social behaviours and bonding dynamics