Learn English – What does “triggered” mean

There is no slang or hidden meaning. She is literally saying that she was 12 years old when the picture was taken.

You are right that it would be more correct to say "I was 12..." However, when you are referring to your past self in a picture they mean the same thing and are interchangeable.

It's either deliberately "wrong" English as slang (for humorous effect) or dialect. There are some English dialects that don't conjugate the verb in a "standard" way, but these are usually spoken not written.

Alternately these might simply be misspellings or laziness. Most of the questions in your search use the construction "what means X?" which is a kind of slang-y inversion of the expression:

X means what?

Note this also is not "formal" English, but I don't think it's considered wrong. The expression "what means X?" is a kind of "baby-talk", though, used to indicate a low level of development or intelligence (like "give cookie!" or "Mommy slow!"), so someone might think it sounds funny to speak that way.

Curious that it shows up on the Japanese language SE though. It could be these are not native English speakers who are also learning Japanese.

My guess is that, if it is slang, it's a passing fad and not something you would emulate. For example, there is a "I can haz cheeseburger?" website that, some years ago, really started the whole "lolcat" and "lolspeak" fad. But if you see a funny picture with the caption "Im in ur foldur keruptin yr fylez" you know this is not meant to be "good" English but just a humorous interpretation of how cats would talk, if they did talk.