Demogorgana is a demon dimension. Scrooge spent an unknown amount of time there during a demon uprising, which was caused by Goldie O'Gilt, who stole the Eye of the Demogorgon, a jewel that makes the wearer impervious to burns.
Due to Demogorgana being timeless, Scrooge did not age during the time he spent there, which partly explains how he could live up to 150 years.