when addressing directly to older peer, you use the proper term for it depending on gender.
For ie -
if a young male addresses to older male - he says "h-yong"
if a young female addresses to older male - she says "oo-bah"
if a young male addresses to older female - he says "noo-nah"
if a young female addresses to older female - she says "un-ni"
and... more headache...
if a nephew/niece addresses to aunt - he/she says "ee-mo"
if a nephew/niece addresses to one's uncle's wife - he/she says "soong-mo"
if a grandchild addresses to grandma - he/she says "hal-muh-nee"
if a relative addresses to uncle (dad's side) - he/she says "sam-choon"
if a relative addresses to uncle (mom's side) - he/she says "kun-abaji"
(and this is barely just a scratch on the surface)
My brother and his friends address me as "h-yong". My old Korean best friend addresses me that way too.
I address my cousins/relatives in Korea as above but to my cousins who are very Californian - I address them by their names.