Mutual liking is more important than age. I know of successful boar bonds of all kinds of ages and age combinations between same age boars and 6 year olds with a gentle baby. The main point is that they get on when they meet!
If there had been a 4 or 5 year old laid back boar in the same rescue at the time instead of 4 months old happy-go-lucky Nye, I would have dated him with my skittish Nosgan without any qualms.
Bonding: Illustrated Dominance Behaviours And Dynamics
PS: If the rescue has got de-sexed sows, you can of course consider a cross gender pairing. Again, liking goes over age. 5 months old neutered Gethin and 5 year old Tesni fell head over heels for each other nearly two years ago - they are still happily married!
My three year old neutered Carwyn was first living with a frail 7 year old lady until she passed away. I then found him a 6 months old and a 6 weeks old pair of rescue sows. It was love at first sniff between daddy Carwyn and his tiny wifelet, who thought him to pocporn and run... But is hopefully shows you that an age difference doesn't need to be an issue.
Generally, the older a boar the less testosterone driven he is; unlike sows who tend to get pickier they older they get, boars mellow with age (exceptions notwithstanding!) It just depends on how dominant the two boys are in outlook and whether they are compatible or not. The key point is - can they meet before you commit?
