Solution

Younger than 250 mio years: Alps, Himalaya, Kaukasus, Atlas

Orogenesis:

alpidic: Alps, Himalaya, Kaukasus, Atlas
variscic-caledonic: Appalachian Mountains
variscic: Ural

Active plate margin: Alps, Himalaya, Kaukasus, Atlas

Mountains lowest in elevation: Appalachian Mountains and Ural
They are the oldest mountains of the list. There is no active plate margin in this zone. Uplift does not take place anymore. In addition, these mountain chains have been deeply eroded. In contrast, young mountain chains are generally still uplifted and show a steep relief.