Generally referring to a younger aged male.
Depending on its use, generally refers to a person who is younger than someone doing the referring, not necessarily gender based either. Meaning it can refer to a female, such as in the film industry where 'best boy' is used, referring to an assistant next in line for promotion.
How the term is used determines the meaning, and does not always imply under age male, in many instances merely is a slang term for someone younger, whether in experience or/and age. Can also refer to a person's level of experience or their submissive position in some cultural hierarchy.
Can also be used in a derogatory sense, as in American Culture, where it was used to denote 'African Americans' in a highly unfavorable connotation. Implications were of being undeveloped, irrespective of the age.
In ancient times, the term was referencing to someone younger, and depending on the society or culture, there were certain expectations made associated with the use of the term. Men had certain obligations, duties, that were not required of 'boys' until certain rituals had allowed them to pass from that stage of life to adulthood.
An example being that when someone, generally male, was considered to be more man than a child, they graduated from wearing shorts to long pants.
Some cultures used the term to refer to those who had reached puberty, but yet were unequipped to assume full manly duties. However the 'age connotation' gradually disappeared in most uses of the term, and referred more to a person's lack of experience, or of being in a submissive role, such as a servant or less authoritive figure, such as 'coxswain' in naval terms. 'Snotty' for example is used in Naval terms as the lowest rank for an officer, generally referred to as Midshipmen who were between the lower ranks and the officer class. It generally applied to younger males, but today simply refers to an achieved ranking. A 'snotty' would still be superior to a 'coxswain' for example, who might have years more experience, but is in a different class.
Generally used to denote a person's lack of experience, as compared to someone older or with more time at a specific position. It is also used to describe a person with less aging effects, such as an older person who has the looks of someone younger, 'boyish features' as an example.
In most instances, use of 'boy' is merely referring to a look, an act, rather than to a specific age grouping.
Sexual references generally refers to someone who is not yet developed as an adult, with facial or body hair. It usually references young men between the ages of 18 to 22, due to their under developed body structure, and presence of facial hair. It is partially used in sexual terms to refer to another's preference for someone simply younger, not necessarily under aged.
'cub' for example, used in the 'bear' culture of sex is merely someone in training, usually younger in age. However not always. Again, it is not age specific.
'twink' is another example where in a sexual preference it is being used to denote one of less age, but not necessarily. It is used to describe a set of standards, that some older males can still meet. The lack of facial hair, body hair for example. Twink however is more 'age' specific, used to avoid negative connotations with one's sexual preferences.
Use of the term 'boy' in a sexual connotation can make some focus on age, rather than a person's attributes. Hence the development of the slang term 'twink' rather than 'boy'.
Used in conjunction with other qualifiers, the use of 'boy' is normally being used to imply a lack of experience, or knowledge in a particular field, such as an understudy would mean in theater language
Primary uses today refer to someone not yet fully matured or experienced in a particular endeavor. It doesn't necessarily refer to an age group.
'boys' is also used to denote a group, again not age specific. Such as the use of 'old boys club' generally is simply about older men who share a common bond, such as the schools they attended, their jobs or such.
It can also be used to denote a 'gender only' connotation not always complimentary. Example being 'boys club' as used by some female politicians. It implies a sexist attitude where the only criteria for acceptance is to be 'male'.