Age of Ruin
Traditions of the Realm
There are traditions in the kingdom that dictate a highborn child's role in life. For the sake of simplicity I'll using males as examples rather than writing "he or she", etc. all the time. In this setting there is no discrimination between the sexes when it comes to who rules.
A firstborn is the heir, or crown prince in the case of the son of the king. He will likely train in both swordcraft and magicianship.
A secondborn is likely to be chosen to develop his magical talents, be they arcane or divine, as well as in the arts of war, should his place in the succession ever improve.
A thirdborn will likewise develop his magical talents, but also concentrate on matters of state, and might one day function as an advisor to the throne.
High born of any place in the succession will have different upbringings should they fail to show any magical aptitude. The path of a knight is still open to them, but never that of a mage knight or paladin. He will be seen by his peers and his people as too weak to rule (although there is precedent for non-magical kings and queens). They often serve as generals and advisors to their more magically adept brothers and sisters.
High born who appear sickly or frail for whatever reason, but display reasonable magical aptitude could be sent to one of a variety of magical or priestly institutions.
Bastards sired by nobles are more often acknowledged than not, and only the oldest and most conservative lords bear them any disrespect. A well-loved bastard is usually raised among the lord's other children, though he is never allowed to take his father's name or crest. Instead, they choose their own heraldric devices, colours and sigils, and are sometimes granted land of their own. Just as often, they choose to enter their lord's service.