Is zakat obligatory on money that I lent to a friend eight years ago, knowing that he is not in need and has not returned it despite my demanding it several times?

The lent money that has remained with your friend for eight years is not subject to Zakat (definition) until it is received. Once you receive it, you pay Zakat for one year. This is the view of the Maliki school. However, some scholars believe that Zakat is obligatory for all the past years, while others hold that a new hawl (Zakat year) begins after its retrieval.