Some people are considered to be a teenager at age 12 but really a teenager begins at age 13 and up.
Once you hear teen in the age then you become a teenager so age 13,14,15,16,17,18 and 19 would be considered to be teenage years even though at age 18 you're considered an adult.
At age 12 you would be considered a tween or Adolescent but at 12 some 12 year olds have hit puberty or they're about to hit puberty.
So at 12 in my opinion you're not considered quite a teenager but you're close to being a teenager.