Page

If one thinks of pages at all, its usually as that cute, or clumsy, boy or girl that trails along behind the great and important people. They are far more than that, even though their duties may be simple and behind the scenes.
If the nobleman or noble lady did not have a page, it would fall to some other servant to run errands, assist with the dressing, receive and carry messages, both public and official and those clandestine. Would any proper member of the nobility really want to trust those messages to a servant with little or no loyalty to them? Pages, both because they are in some ways, hostage to the houses they serve in have a vested interest in making sure the people they serve are accommodated according to their desires. A page who behaved poorly or violated confidentiality might not just be beaten, but their family could be at risk. 
A page to a spymaster might begin to learn to acquire materials other wished to keep secret. They appear to be an innocent youngster running a typical errand for their master, but that innocence can be a great tool and shaped well by one who needs such a boy or girl. For a page it would be a grand adventure.
The librarian page learns to read and write very well, to understand organization methods and to think critically. They develop reasoning skills that will stand them in great stead in the library or elsewhere as they get older. They are often the first to make interesting connections with information as they are not set in their ways and are not afraid to be imaginative in their thinking. Properly developed, such pages become exceptional adults that can wield great influence.
Page are important, not just for what they bring during their service, but also for what they become because of it.

Career

Qualifications

Minimal qualifications. Ages 7-10 to start. Able to walk, run, see and hear and understand and obey orders.

Career Progression

Pages typically age out in their mid-teens. Depending on where their service as pages takes place, they may move into more noble training, to be a squire, or lady-in-waiting. Or they may become an assistant or clerk to a librarian or a judge or other member of the government.

Payment & Reimbursement

Small payments may be made for service above and beyond, but most pages receive room, board, and livery as they are expected to dress in the uniform of the place where they serve.

Other Benefits

Training in reading, writing, organizational structures.
Use of arms and courtly manners
Stealth techniques, observation skills