In Schleswig-Holstein, we have mountains that are 20 meters above sea level. And maybe 10 meters above the surrounding landscape. Though while they have "berg" in their name, I think everyone would still call them a hill.
When RPG maps indicate an area as mountains, my assumption is that its side are too steep to have a closed cover of vegetation. Though of course lack of water or lack of soil can still get you hills of bare rock as well.
My rule of thumb would be that you have to climb a mountain, but you hike up a hill. Though again, some conical volcanoes have very flat and smooth slopes that you can walk up to the top but are still absolutely massive in size.
Even as a Sea German, I would say that the Brocken in central Germany is a big hill and not a mountain. But Ben Nevis in Britain certainly looks like one, even though it's not a lot taller.