Hesitant To Flirt With Girls

So all my life I always assumed I looked my age, I never though I looked younger than I do until everytime I say my age people always get surprised and say "what!" At first I thought people were just bull shitting me or just being nice but it really started to show when 19 year Olds & 20 year Olds starting flirting with me O.O. now eveytime I see a girl approach me I hesitate.