Matt Birk announces retirement

Matt Birk has announced that he is retiring from playing after 15 seasons in the National Football League.

The Baltimore Ravens centre has decided to end his career ahead of the 2013 season.

While giving a talk at a local school, the 36-year-old said: "I've enjoyed playing football. But as much as playing football, I've enjoyed doing this. I'm old. I've got six kids. It's time to do something else."

Birk has spent the last four season with the Ravens, having started his career with the Minnesota Vikings.