Why does master mean 'man' and 'boy'?

Why does master mean both 'one having authority' and 'a young boy'?

Merriam Webster definition of 'master':

One having authority over another

A youth or boy too young to be called mister

The eldest son of a Scottish viscount or baron

Oxford Dictionary Online definition of 'master':

A man who has people working for him, especially servants or slaves.

A man in charge of an organization or group.

A skilled practitioner of a particular art or activity.

A person who holds a second or further degree.

Used as a title prefixed to the name of a boy not old enough to be called ‘Mr’

a. A title for a man of high rank or learning.

b. The title of the heir apparent of a Scottish viscount or baron.

_{The definitions in italics are supplied for context, since that root might shed some light on the issue.}

Etymoline etymology:

late Old English mægester "one having control or authority," from Latin magister (n.) "chief, head, director, teacher" (source of Old French maistre, French maître, Spanish and Italian maestro, Portuguese mestre, Dutch meester, German Meister), contrastive adjective ("he who is greater") from magis (adv.) "more," from PIE mag-yos-, comparative of root meg- "great."

As you can see, the word has both 'senior' meanings of a superior, someone who has mastered something, someone with a Master's Degree, etc. as well as the 'junior' meaning of a young boy, even though the roots are words like 'he who is greater'.

Also, the Oxford Dictionary Online includes 'A title for a man of high rank or learning' under the definition of 'a boy not old enough to be called ‘Mr’.' In fact, a man of high rank or learning is clearly different from the 'young' master - the man could even be teaching the younger 'masters' (think 'schoolmaster', 'headmaster', etc.)

Questions:

Why does 'master' mean both a man and a boy?
Where did this juxtaposition of Oxford definitions come about?

Solution 1:

As suggested by the following extract, the original usage of Master dates back to Old English:

The original sense in English is that of a man with authority or control over others. It appears as early as c.897 in Alfred’s translation of Gregory’s Pastoral Care:

"Thonne he gemette tha scylde the he stieran scolde, hrædlice he gecythde thæt he wæs magister & ealdormonn."

It later developed also into a title of respect, which was later replaced by Mister:

Master was used in England for men of some rank, especially "free masters" of a trade guild and by any manual worker or servant employee addressing his employer (his master), but also generally by those lower in status to gentlemen, priests, or scholars. In the Elizabethan period, it was used between equals, especially to a group ("My masters"), mainly by urban artisans and tradespeople. It was later extended to all respectable men and was the forerunner of Mister.

The earliest usages as a title to refer to a boy dates back to the early 16th century:

The use of master as a title goes back to Old English. But the use of master as a title for a boy or a young man is also from the early 16th century. From c.1533-34 in letter by an H. Dowes:

It pleased your Maistershipp to give me in charge not onlie to give diligent attendaunce uppon Maister Gregory.

This usage as a title for boys survived though centuries also after Mister was introduced, even though it is now considered archaic:

After its replacement in common speech by Mister, Master was retained as a form of address only for boys who have not yet entered society.

By the late 19th century, etiquette dictated that men be addressed as Mister, and boys as Master.

Current usages:

In the United Kingdom:

The use of Master as a prefixed title was, according to Leslie Dunkling, "until recently ... a way of addressing politely a boy who was too young to be called 'Mister'." It was used as a title for the eldest son only; younger sons had no form of address.

In the United States:

Nancy Tuckerman of the Amy Vanderbilt Complete Book of Etiquette writes that in the USA, unlike the UK, a boy can be addressed as master only until age 12, then is addressed only by his name with no title until he turns 18, when he takes the title of Mr., although it is not improper to use Mr. if he is slightly younger.

(Wordorigins.com/Wikipedia)