While most if not all military organizations will train recruits, some skill and/or aptitude is typically required before training so that less time and resources are wasted on training someone who won’t be up for the job’s rigors. If we’ve already decided the expected skillset of accepted members (or ones that have completed training), this can help us decide on the prerequisites. For example, if advanced horsemanship will be acquired during training, basic horsemanship is a prerequisite.
Some level of proficiency with various weapons is likely required, and these can be divided into broad categories: long and short bladed weapons (swords and knives), blunt force weapons, and range weapons (bows, guns). Elite groups may require more skill across a wider array of weapons. We can decide that if there are six requirements and someone only has four but shows great promise in those, that they are provisionally accepted. There are no rules except for those we invent. Some organizations will churn out elite fighters while others may only produce average warriors who are half-expected to die within two years of enlisting.
Not all prerequisites involve something physical. Knights may require noble birth. Perhaps they must show strength of character, which may either be tested or vouched for by reputable sources. There might be educational requirements or the ability to read and write certain languages. This can make characters more believable, especially if they aspire to belong to this group and must prepare for an initiation test. If they’ve failed once too often, maybe they cannot be considered anymore and have feelings about this, ones we can exploit. Remember that some underprivileged people might try to join the military precisely to gain access to things they can’t otherwise acquire, such as food, lodging, clothing, and pay.