I think I've figured out Annie's age.

Okay so Annie had a quote when she was released, said quote was "I'm five!", that would mean she was released on her 5th birthday, as her birthday is February 2nd aka her release date as confirmed by the Riot Team, so add a year to her age per-birthday and it's quite simple since her release, aka her 5th birthday she has had 6 birthdays, since she is getting older as confirmed by Riot that would mean she is at this point in time 11 years old. This also fits with her redesign a while back, most people said she looked about 8 after it and about 5 before it, this fits as Annie was now 8 years old, as the redesign came 3 years after her release, and therefore her 5th birthday, hence the redesign making her look older. So this is why i believe Annie's current age to be Eleven.

(Personally I hope riot keeps making her look older with updates but that would take a lot of work and commitment, and it may better suit their purposes to leave her untouched as is now till she really needs an update.)