IMC, it is custom for nobles and the well-to-do to group their children of similar age together in groups known as "parties". From 7 through 14 they live and educated together until the begin training in their trades (ie classes). The idea is that this is a known group of friends that have grown up together whose fortune are intertwined and will help each other later in life.
For example, the prince of a kingdom may be raised with the children of the head priest, court wizard, and captain of the guard. Most of which are intended to inherit their parents postion later in life. Then, when the prince becomes the king, the new head priest, court wizard, and captain of the guard are life long friends who are trustworthy and will support the new king. In many cases, it is considered desireable and good luck to have a "man of the people" or non-noble in such a group so that this person may garner the support of the commoners. This is sometimes but not always a rogue.
In many cases, such parties are all that a well-to-do family or even noblity can offer the youngest of many children. While the children will inherit the titles and fortunes of the family, providing such parties for their younger children to be in so that they may attmept to support themselves once adult is seen as a decent inheretance. Many adventurer parties come from such groups of families who can afford to provide training for their children but no actual jobs. This is where most 1st level PC parties are seen to come from, and thus they have known eachother for most their lives and expect their fortunes to be linked.
