I recall a law that states in America/Canada which states something along the lines of “judgement by a jury of peers”.
So, does that mean a teenager would be judged by people his age?
No. In the egalitarian U.S., all citizens are peers of each other. This is in contrast to, say, medieval England, where one might be a commoner, a noble, or royalty, with his peers being other members of the same group.
The various states may, and do, have age requirements for jury service, just as they do for voting. In fact, most states use voting rolls to select names for jury duty.
They also use DMV records to get people who aren’t registered to vote.
By the way, the trial-by-jury doctrine, which comes to us from the good folks in England, is codified in the Constitution thusly:
No. The phrase “jury of peers” derives from the Magna Carta (1215); it is a loose (but not inaccurate) translation of a phrase more precisely rendered as the “judgment of equals”:
The term “peer” or “equal” as used in the Magna Carta had not yet taken on its modern sociological connotation, in which a “peer” often refers to a “peer group” of a common age; instead, a “peer” was simply “a person who is of equal status, rank, or character with another.” Black’s Law Dictionary 1152 (7th ed. 1999). The word still holds this meaning, for example in the context of “peer review”–that is, review by colleagues or experts in the same professional discipline. The “equality” involved in the “judgment of equals” meant equality of social status, so that a commoner who was accused by a noble of an offense was entitled to a trial by his or her fellow commoners, rather than by the noble accuser’s cronies. As Blackstone explains:
1 William Blackstone, Commentaries on the Laws of England 391 (1765).
The phrases “jury of peers” and “judgment of equals” do not appear in the Constitution of the United States. But the Magna Carta’s article 39 does survive in recognizable form in the fifth amendment:
The concept of a “jury of peers” falls within the concept of “due process of law.” Because the new United States was a classless society, which constitutionally prohibited a noble class, the need for spelling out a “jury of peers” disappeared because everyone in the new society (well, at least all adult male freeholders, at first) was supposedly equal in the law’s eyes.
I see that Gary T and friedo beat me to it. Serves me right for not previewing (but then again, why waste all that typing? :)).
Which is also why, until recently at least (I don’t think that it’s still the case), in the UK, someone with a title of nobility who was accused of a crime had the right to be tried in the House of Lords.
But your answer was far more complete and informative, so don’t feel bad. 
Ah, but the additional depth in your reply was most welcome.
In Canada, the Young Offenders Act was enacted in 1984, to regulate the trials of anyone who was under 18 and charged with a criminal offence. There was no provision for jury trials at first. That was challenged unsuccessfuly under the Canadian Charter of Rights and Freedoms. The courts held that since a young person could not be sentenced to more than five years, the lack of jury didn’t breach the jury provision of the Charter which reads:
The Act has been amended to allow for punishments over five years, so now there is a provision for jury trials, but the jury pool is composed of those over 18, so far as I know.
In the U.S., the accused has a right to a jury trial for any felony, which is typically defined as any crime for which the maximum sentence is more than one year imprisonment.
–Cliffy