Oh shit...
Seems like the author messed up whoops
Could be a translation error, or that old guy didn't know what he was talking about? I think it's more likely he didn't know what he was talking about, if we saw him tell him that Nam is the oldest we wouldn't assume that guy was Nam's older brother. Maybe to throw us off the trail?
He said the oldest one at the bathhouse. So not oldest outside the bathhouse. ╮(￣▽￣)╭
but I thought he was the eldest son http://www.mangago.zone/read-manga/following_namsoo_to_the_bathhouse/mh/c002/5/