Throughout his childhood and high school years, Seifert resided with his mother and two brothers on the Danish island of Bornholm. From the age of 11, Seifert spent summers and Christmas holidays in San Rafael, California with his father, who immigrated to the United States and founded the microcomputer products firm Sun-Flex Company, which was later sold to Xidex Corporation. There, his father co-invented an anti-glare device for computer terminals, which was awarded United States Patent number 4,253,737 in 1981. It was during the first summer holiday with his father that Seifert became interested in computers and wrote his first computer program. At age 15, with the help of a friend and his mother, Seifert started his first IT company, Danbyte, which imported computer disks to Bornholm.