personally I think Oro and kakashi have some sort of history... maybe in the anbu days or something. But they seemed much more familiar then say, his greeting to Itachi.
and I hope your not saying that kakashi would be weaker in 20 years.... as far as we can tell, the only age hindered ninja was sarutobi, and he was 80. I think kakashi's about 20, so in 20 more years, that'd but him at least 10 years below ero-sennin, who is still fighting strong.... tho, to contradict myself, obviously age is a factor there, because oro has switched bodies once already. Tho that could have just been testing the jutsu.
I'm a little indecisive this morning