As you may have known, the 2015 FIFA Women's world Cup Final will be USA vs Japan again. Anne Hathaway on her blog told the US women's team that "I am 12 in this photo" And this reminded me of a urban slang, "I'm 12 and what is this" on Urban Dictionary. I am very sure that Anne Hathaway is not 12 years old now. She is older than me. I don't understand why she did not write "I was 12 in this photo", but "I am 12 in this photo".
Could you explain this to me please?
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.
If you are talking about the photo found on this page, which is of a twelve-year-old Anne Hathaway dressed in a soccer uniform with the number 12, notice also the comment by Hathaway is
In honor of the U.S. Women's Team who has made it to the FINAL of the World Cup, here is my #fbf football photo. Ironically, I am 12 in this photo. USA! USA!!
then first and foremost, and unambiguously, she means she was twelve years old when the photo of her in a number 12 jersey was taken.
You can use either the past tense or present tense to describe how old a person is in a photo. An extension of this principle is that the present tense is often used for captions of photos, whether in a book or a newspaper. This is because the thing in the photo is doing the action at the time the photo was taken. Or, as far as age of a person is concerned, the person is that age in the photo.
Still, it would also be okay to say "I was twelve in this photo."
The main thing with Hathaway's photo is that in posting it on the web, she is showing her support for the team. But unless she has specifically stated this elsewhere, any reference to her bring the 'twelfth player' is a stretch. The irony that Hathaway actually mentions is that in the photo in which she is wearing a number 12 jersey, she also happens to be twelve years old. If she had been wearing a number 92 jersey in the photo, she would still be showing her support, but the irony of her being the same age in the photo as the jersey number would be gone.
This is not referring to her age, but a fictional player number:
A football (BrE) / socccer (AmE) team has 11 players on the field. The fans are often called the 12th player, partly because they identify with the team, partly because fan support can really help a team play better.
If you look at the photo at instagram, she's wearing a team outfit with a 12 on her chest. She's showing her support for the US team.
To those less familiar with the numbering system in football/soccer:
Each player of a team gets his personal number, at least for a season, more often for longer. As a team has more than eleven players in total, the numbers are higher than only 1-11, but there will always be 11 (or less in case of penalties or injuries) players on the field. This is why in some languages a team is referred to as "an eleven" (German: "Elf", Dutch: "Elftal").
