If I forgot my age I'd go look in the mirror. If I hadn't shaved, then probably go for my current age, which is 24. However, if I was clean shaven, then 18 या 19. I never get asked for ID when I have a beard, but get told I look like 18 when I don't so go figure. Haha.
Well, let's think. Ones age would remain the same regardless of their knowledge of what it was. So, if I did not know that I was 15 then BAM! I would be 15.