Squire

A Squire is essentially a knight in training. In more slightly detail a Knight's career starts as such...

• First, you're born. Then you're a baby and hopefully you last through your first three years. Many medieval kids did not. Normally the sons of knights become knights themselves, but sometimes commoners get taken on if there is a shortage of sons of age.
• Once you're a toddler, you get raised and educated by your mom and her servants, learning the basics.
• At age Seven you get sent off to be a Page. You serve as a servant at a castle, do your schoolwork and are run through the basics of combat training. This continues for seven years.
• At age 14 you graduate to being a Squire. You are assigned to a Knight who shows you the ropes while you follow him around in the field, tend to his horse, cook his food, keep his armor in good order and do similar stuff while practicing with weapons, horseback riding and similar. You might have to fight, but generally you avoid that. In modern military jargon, the Knight is a Tooth and you are the Tail. This continues for seven years.
• At age 21 you graduate to being a full fledged knight. You get Knighted by your liege lord.