Bastion/WMG

The Kid's Age
Rucks states that the Kid joined the military because "School ain't workin' out." Assuming shared school-ages between Earth and Caeldonia, he would drop out of highschool (age 17), opt out of college (age 18-19), or drop out of college (19-20). That gives us a four-year age-range. After his five-year tour (I think it was five), that puts him at 22-25 years old. His second tour was interupted by the Calamity, which gives us a range of 0-5 additional years, but more likely 1-4 (at least one full year served and at least one full year left). This give us a final range of 23-29, not a child by any means, but still young enough that fifty-or-sixty-something Rucks would be justified calling him "Kid."
 * My guess is high-school drop-out, serving a four-year second tour. 17 + 5 + 4 = 26
 * I'm going to go with a younger age, the world of Bastion seems to be in the industrial age, where it is not uncommon for a kid at the age of 12 to leave school. Give him 9 years of service, that puts him at 21 for the game, plenty young to be called "Kid."
 * That was how I took it, too...but I figured he'd spent closer to a year or two into his second tour of duty, making him more like 19, tops.

While it's not explicit, when starting a New Game+,

This could easily suggest that while


 * Or, due to  The Bastion will never have to send the survivors all the way back into the past before the calamity. Even in New game +, Rucks will make comments about already saying things or having dejavu on saying them, so the memory function is at least partly functional just like the

Rucks is related to the Kid in some way
Possibly even his father. Both have white hair, even though Ruck's could be from age it seems to be implied that white hair is unusual in Caelondia as both the Kid and his mother were ostracized for it. Also how coincidental is it that the  is alive and so is this one other guy who happens to have the city crest which is somehow connected to. Somewhat related: the first time I heard about the Bastion's, I almost immediately thought Rucks would turn out to be.
 * However, Rucks is old enough to have come by his white hair honestly.
 * And we're specifically told that "everybody on official business for the City carries the Star of Caelondia," so that's not so useful.
 * The question then becomes, since the Crest protected The Kid, and probably causes the "land forming under his feet" effect ("there's a bit of the Bastion's power in the Crest," Rucks tells us), why didn't anybody else's Crest work like that?
 * The Kid survived because he was scouting the outer edges of the Wall, not because of the crest. Zia and Zulf survive because they sheltered in Ura Dens. Only Rucks' survival is unexplained in the game. Presumably, anyone's crest would work, as long as they survived the Calamity.
 * Rucks was able to survive due to already being at the bastion before the calamity happened.

All replays are canon
One of the endings allows you to

The Kid is the son of Asura
White hair, uncanny determination and battle dialogue composed almost entirely of enraged grunts.

There are other tribes of people out there in the world who survived.
The Kid, Rucks and co. will link up with them. Though this is more of the optimist in me speaking than anything from the game.
 * Don't be so negative about that - After all, the background during Personally, I think that they
 * Not to mention that hey, an awful lot of Ura survived the Calamity and it was specifically designed to kill all of them. Granted, they did this by burrowing underground, but surely some people farther from Caledonia would do even better.
 * Also, it should be noted that the Calamity
 * It's almost explicitly stated that Caelondia is a colony, presumably on a different colony then the Cael homeland. Since distances are never mentioned, and the only other city (If it can be called that) Kid visits is the Tazal Terminals, it seems fairly reasonable that only a (relatively) small area of the world was affected. After all, the.

Zia planned to betray the Bastion to the Ura survivors, while Zulf was completly sincere
Think about it; The game pretty much spells out Zulf was a diplomat, so making friends with Caelondians was his job. He most likely planned to get back in touch with the rest (now that ) and broker a peace deal between the survivors. When

Meanwhile think about Zia's motives; before the calamity  While it's possible she didn't hold a grudge (since she was culturally Caelondian anyway), keep in mind that her song/theme is basically (well, see my next WMG, but still) a threat aimed at Caelondia from the Ura. Therefore it's possible that

The song Zia sings actually a lot less warlike than it sounds.
Reading the lyrics it sounds a lot like a threat from the Ura to Caelondia. However when you think about the double meanings in the other songs with lyrics it could just as easily be a warning (after all, if the Ura had religious beliefs against living above ground and they were on good enough terms to send a diplomat they might be inclined to "warn" the Caelondians. It could also imply an intention to build stronger ties; "So build that wall and build it strong/'Cause we'll be there before too long" could mean "Make sure you're well defended, because we're going to attack" or it could mean "make sure you've got some good housing sorted out for when we join you".

Rucks is actually .
He seems to be the only one to have partial memory of the events occurring, and tells the full story to Zia, even parts that he probably wouldn't have heard otherwise. Because of the damage to the Bastion, instead of sending back all of the occupants with their memories intact, only the Kid (the one who pulled the switch) makes it back to Caelondia about twenty or thirty years before the Calamity. They dress nearly the same, they have the same hair and (from what I can tell) eye color, and Rucks seems to be vaguely aware of the workings of the time loop caused by restoration.
 * Wait a minute. If The Kid gets preserved in each Restoration, how did we end up with two? Also, Rucks advises The Kid in the game... if he knew that much, wouldn't he have fixed the problem when he was still young enough to get around?

The Kid is
Based solely on the Groundhog Day Loop nature of the game's plot, and the fact that the kid can equip himself with some badass revolvers. There's not enough evidence otherwise; this theory just seems rather cool.
 * Alternately, The Kid is.

Rucks wasn't lying about his careers.
He's at least 20 years older than the other 3 main characters. So who says one can't move up the ranks of an organization? Maybe the reason no one expects someone to take multiple tours of duty as a Mason is because they expect you to either retire or change career paths. He could have started as a Mason, moved up to being a Trigger.
 * When choosing the Cael Hammer/Army Rifle combo he comments that this is his own favourite set from the past. Given that both weapons seem to be EXCLUSIVE for a particular guild (The Masons and Triggers respectively) it's possible that he worked his turn on the Walls, then enrolled into the Army.

Rucks knows Cave Johnson
Because I have added this moment of epicness to my own personal canon.