今年 means this year, the point is when people use 我今年十五岁，Literally "I'm 15 years this year. We are used to put 今年when we talk about age .When it comes to age, there are two ways that Chinese people count age, 虚岁and实岁，people would add one year on their ages right after spring festival when they use 虚岁, and people would add one year after their birthdays if they are used to 实岁。therefore, that depends on the way people counting age, it has nothing to do with 今年.