The object of every horse race is for each and every competitor to "win". However, it is not always the best horse that does in fact win the race.
There are a number of different horse racing variants that are designed to "even" the field and find a winner at level stakes.
The two most popular methods are handicap and weight for age.
In a handicap event each horse is allocated a penalty weight according to their previous performances. Thus, theoretically, on previous form all horses should be racing at an even level.
The second variant, weight for age, allocates all horses a differnet penalty weight based upon their (the horses) age. This theory centres around the fact that horses peak in their abilities at a certain age. This age is open to interpretation and clearly changes by horse and distance. Many people think it is around 5yrs of age.
Nonetheless, many weight for age races historically bias three year old horses, supposedly at the peak of form over medium distance (1 mile for example).