I'd say up until about age 14 or 15, the average American youth doesn't have any secrets worth keeping. The whole point of the magical "dear diary" is to pour out the superficial embarrassments and heartaches of adolescence, oblivious to the utterly transcendental nature of the competing pressures and fear of failure, and the difficulty of attempting to reconcile the need to affirm an identity distinct from others yet compatible with the limited society of one's exposure. The incompatible drives summed up well as "You can't understand how I feel!" and "Isn't there anyone who understands me?" Hormones don't help a whole lot.
Before tipping over into the juvenile portion of adolescence, however, the "tweens" and younger, life is a much simpler power struggle within the sibling pecking order. I can easily recall the formidable vehemence with which secrets were guarded, and the deviousness with which they were created solely for that purpose. I cannot bring to mind, however, the contents of those secrets. I'm sure they were naively inflammatory, along the lines of "so-and-so is mean" or "smells", or otherwise possesses objectionable qualities or exhibits diminutive aptitudes. These pithy epithets would then be lovingly translated into an immature cypher and safeguarded after some grand parade or announcement of their existence was put on.
The only reason the pseudo-clandestine article held any draw for the target subjects is well illustrated by a quote from The Simpson's:
Lisa: Dad, this isn't about glue. It's about territoriality.
He only wants the glue because I'm using it.
Bart: Oh yeah? Prove it.
Lisa: [hands him the glue] Here.
Bart: Hey man, I don't want your stupid glue.
Bart tosses the glue away
-- Bart vs. Thanksgiving (7F07)
Assertion of control over the artificial restriction reaffirms one's importance and viability in the competition for... whatever it is we were competing for.
The method of obscuring the text was invariably a form of substitution of letters for other letters, numbers, or symbols in a one-to-one relationship. Elaborate maps of the enigmatic keys would be refined and hidden, and the correspondingly enciphered material could then be allowed to fall into enemy hands in the knowledge that the sibling(s) would encounter prolonged frustration in attempting to comprehend the contents, and if eventually successfully would only be annoyed at the revelation.
The most amusing one I ever encountered was concocted by my sisters: the holy grail, the impenetrable Double Cypher. This involved substituting the letter for a number - and then that number for another number. The mathematical relationship of A = B = C being identical to A = C not only escaped attention, it resisted enlightenment when confronted.
With enough practice at the game and adequate source material, any simple substitution puzzle can be solved (forming the basis for the Sherlock Holmes' novella The Adventure of the Dancing Men) on its own. Once my brother and I entered determinedly into this altitude of the arms race it was time to move on to new forms of subterfuge.
Later, my older sister sought to confound the process further by introducing phonetic constructs. But by then it was outside of this petty rivalry and instead used to foster the darker side of Borderline Personality Disorder, allowing a descent into paranoid dysphoria in the loudest silent way possible - writing on walls of her room and all over notebooks in an intentional display of manipulative privacy: you weren't supposed to know what she was thinking, but you were supposed to know that she was thinking and it was bothering her (thus allowing for a projection of her emotional caretaking needs onto others without their explicit consent or even involvement, and without the insight required to fulfill the heinous responsibility). Heaven help he who dared trod on the meanings of said script, too.
I eventually inherited the room for an office (having moved into my Father's house by then) after she moved out and on with her life, and I took the time to decipher the scrawl before returning the walls to neutral colors. The thoughts were angry and hurtful, frequently dictating the dire consequences to issues outside of her control were they to resolve in a light unfavorable to herself (things that would "happen" or that she would do if she were not allowed to X, for example). Things that no one should have to suffer alone, real or imagined.
Not that there's much point to my anecdotal ramblings - childhood was carefree even in most of its assaults, people should care about and help one another, and it's pretty pointless to hide those things which give us cause and identity (as by doing so the possibility of caring connection is mitigated).
Today I suppose I've carried that into a general philosophy: aside from necessarily hiding the financially sensitive and authoritatively identifying bits and numbers assigned to me as a modern citizen, there's nothing in my life I choose to hide away from the world. Though some of this is probably the cynicism that the world wouldn't care to do anything about those things I so freely share anyway, being unimportant and uninteresting on any grand scale. Randomly useful perhaps, but not actually interesting.