Garp Respected Roger. Not liked. Just as Smoker respects him.
That can be Smoker too. He's really close to Egghead after all.
Age doesn't matter at all. Portrayal does.
Doesn't matter. Roger didn't have a nick name as far as we know.
Coby is Luffy's friend dude. Always has been. Always will be. At best, they will have a friendly fight in the epilogue, when coby is an admiral just as they promised.
Age actually does since none of in the verse that was portrayed with someone else much older than that other person.
Bigmom and kaido. Aren't 20 years apart.
Shanks and mihawk aren't 20 years apart
Garp and rodger aren't 20 years apart.
Whitebeard garp aren't 20 years apart.
Whitebeard and rodger aren't 20 years apart
Rayleigh and snegoku aren't 20 years apart.
Kidd law luffy aren't 20nyears apart
That's the narrative
To add rodger also liked garp and told him that he trusts him as much as his crew. Hence the same dynamic with luffy and his crew and luffy and coby. They are the few people luffys calls by name. His crew and coby.
He does not show thay same level of respect with Smoker.