The Humboldt University of Berlin is illuminated during the 10th annual Festival of Lights in Berlin in 2014. (Paul Zinken/European Pressphoto Agency)

Franz-Josef Mittnacht likes to be considered as just an average student.

But in one way, he really isn't that average: Mittnacht is 86 years old. He is among a growing number of European retirees who have flocked to university campuses to earn degrees.

"When I'm 90, I want to be awarded a PhD. I'd like my tombstone to read: 'He studied until the very end,' " Mittnacht was recently quoted as saying by Germany's Der Spiegel magazine.

Between 1981 and 2011, the number of retirees enrolled in college rose by about 500 percent in Switzerland, according to the country's statistics office. Other European countries have seen similar increases.

Many European universities do not charge tuition, which makes it easier for retirees to study. Moreover, many colleges do not require students to go through a selection process and instead just allow them to sign up for certain programs. Such conditions have made it easier for retirees to enroll.

But why has earning a degree at the end of a career become so popular?

Many retirees enrolled at European colleges say that studying has given them an opportunity to engage with others more easily. "The lectures are like a window to the world for me," 72-year old Ute Ulrich said in an interview with Swiss magazine Beobachter.

"In order to keep your brain fit, one has to meet two criteria: You have to challenge yourself and you have to engage socially," said Ulrich who used to work in the tourism industry. Now, she is focusing on the Arab Spring. At her university in Zurich, more than 160 retirees are enrolled in degrees.

Mental health researchers increasingly consider studying as a long-neglected healthy exercise for retirees. "Many want to reorient themselves, to fulfill a long-held childhood dream or they simply want to continue their education to use their time effectively," according to Studi-Info, a German website focused on counseling students.

At many European universities, it is also possible to attend lectures as guests or to participate in particular "pensioners' schemes," which are created for older students.

Interest in such courses has not gone unnoticed in the world of private higher education. In Germany, students can now also attend a university specifically for retirees in Bad Meinberg. They can choose among courses such as medicine, philosophy or political science.

However, not everyone is completely happy with retirees storming European campuses. According to some student organizations, older students have overcrowded regular courses and even forced younger students out of some of them. Consequently, some universities have prohibited retirees from signing up for certain popular courses over the last few years.

But nevertheless, interest is growing.

Mittnacht, who is now studying Medieval History at the university of Duisburg-Essen, told Der Spiegel that studying has given his life new purpose.

He says he has never been happier.

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