Each year, Assen hosts the Jeugdtour - the world's biggest youth cycling event. This takes part on various different routes in and around the city over a week. In 2008 there were 700 competitors from many nations.
This video shows a criterium in the east of the city, followed by the Dikke Banden race (fat tyre race - i.e. mostly very young children on non racing bikes).
There are also time trials and a "classic" of around 100 km in length. Competitors range from about 7 up to 19 (and from around 5 in the case of the dikke banden race).
In general the races are run on closed roads, though they also use the cycle racing circuit and some cycle paths such as those through the Asserbos.
While this is a youth event, it's also a serious event. One of the Swedish competitors went on to get a silver medal in the 2008 Olympics.