I started cosplaying (meaning planning to work on a cosplay) November 2009 so, uhhm.... I was 16 years old. I'm 17 right now, soon 18.
But if I don't remember all wrong I attended a small "manga day" event before that, ahaha.
*~Cosplays in progress~*
Masamune Date - Sengoku Basara
Eren Yeager (casual outfit) - Attack on Titan
Levi (suit) - Attack on Titan