I've always been puzzled at many countries allow teenagers to have sex with each other at 14-15 years old at the youngest but still forbid them from consuming intoxicants until they are 18 or 21. Why do they have some bodily rights but not others?
There is and never has been a single “age of majority”
The law treats people of different ages differently for a wide variety of reasons. Just a small sample of typical limits (jurisdictions vary):
- you must be 4 to go to school
- you must be 10 to be criminally responsible
- you must be 11 or 13 to get a job in the australian-capital-territory but there is no specific age limit in new-south-wales
- you must be 15 to attend an M-rated movie alone (your rating system may differ)
- you must be 16 to get a learner driver permit and 17 to get a provisional licence
- you must be 18 to attend an R-rated movie at all
- you must be 14-17 years old to have sex depending on the age and relationship with your partner.
- you must be 16 to 21 to buy and consume alcohol (jurisdictions vary a lot on this)
- you must be 18 to marry without the consent of the court but can be as young as 16 with that consent
- you must be 18 to legally take and post sexually explicit pictures of yourself
- you must be 18 to get a tattoo
- you must be 18 to join the defence force
- you must be 18 to vote
- you must be 35 to stand for President of the united-states
So tell me, when is the age of majority. There isn’t one - there is simply a long period where an individual is granted progressively more control and responsibility over their life.
Now, as to why some things happen at particular ages, that’s a political question, not a legal one.
This is likely due to the fact that until relatively recently marriage usually occured relatively early in life. Consider the scene in "It's a Wonderful Life" where George learns the "horrific" fate of his wife Mary if he never was born: She's "an old maid" who never married. In the film, Mary's fairly young in portrayal in 1945 (George is 38 years old and Mary is the younger sister of a classmate of George's) and their marriage is when George is 21 (Mary would be at most 20, as would be George's younger brother Harry, who marries before George.). Considering the year depicted at this point in the film is 1928, it shows just how young marriage age was in 100 years prior.
Before that it wasn't uncommon for people to marry in their teens, to the point that it's accepted that (a different Mary) was 13 years old when she gave birth to Jesus Christ and she was about to be married when she became pregnant (by the spirit of God).
As for the law, the Common-Law, which does not like to overturn past decisions, has had the age of adulthood to be at 16 for the purposes of marriage for sometime, which is why most Common-Law countries are 16 years old (In the United States, the majority of States are 16 years old, however it's 18 by the Federal Government (which only comes into play if the relationship is over state lines) and California (among others, but it is where most of the world's porn is produced).
In addition, many countries have a "Romeo-Juliet exception" to their statutory rape laws which allows for a little wiggle room for a couple where the age is relatively close but the younger is not yet past the age of consent while the older member is past the age of consent. This is so the couples can, um, couple without having a criminal record (and to protect the older member if the Junior member of the pair falsely charges rape to avoid admitting it was consentual to an outraged parent). The protection does not cover a couple where the older person is in a position of trust (a teaching-aid/student teacher, a coach, ect.) over the junior member of the relationship (it's not unheard of for these aids to be fairly close to their student ages in some settings).