"Oben sind doch nur Erwachsene, warum sollten wir nach oben gehen?"
Personally I would sooner translate "oben" as "up there" instead of just "upstairs" - is that wrong?
It could mean "up there", but if the speaker did actually mean "upstairs" he most likely would use the german sentence as above.
Thus, it seems more likely that the intention was "upstairs", but it is only clear from the context.
But with no context given (as here) would you sooner think "upstairs" or "up there"?
Upstairs are however only adults, why should we go upstairs? was marked as wrong, where is 'doch' then?