What are conditions (restrictions) in horse racing?

Conditions are restrictions that limit races to certain types of horses.
There are different types of conditions. One of the most common restriction is that a race is limited to female horses – fillies and mares, depending on their ages. That means no male horses are allowed to run in a particular race.
Another common condition is an age limitation. Some races, for instance, are limited to 3-year-olds, which prevents more-seasoned horses from running against younger, less developed horses.
Conditions can get quite complicated. For example, a race might be limited to horses that have not won a race worth $10,000 since Jan 1 of the preceding year, except for maiden, claiming or starter allowance races or not have won two races in its career.
Some racing secretaries get very creative with conditions. Emerald Downs racetrack in Washington state recently carded a race where the trainer of an entered horse had to be at least 90 years old.
Reading the conditions of a race is a good starting point when you begin handicapping, as you may discover that a certain horse fits the conditions perfectly, while another is entered at a higher level than it is eligible for.