Even if, let's say, SM is 20 and SK is 8, SM is 22 and SM is a teen, or if SM and SK are only a couple of years apart?

What should the decorum be between a SK and SM who are far apart in age?

Here's what I think for each situation I mentioned:

A) SK at 8 and SM at 20 and SK at teen years and SM at 22:

SK should show SM the same respect and courteous behavior they'd show a teacher. We all know there are lots of younger teachers who are fairly close in age to the kids they teach. Most parents expect their kids to treat said teachers with respect. Why should it be any different for SMs?

For SMs and SKs who are all adults and close in age, I think both parties should treat each other with the same respect they show other fellow adults. At that point, they're all equals. The SKs should treat the SM with the same courtesy they want their dad to show their partners, even if their partners happen to be close in age to him (dad).