Hmm, I first tried when I was like three or four. Tried to say 'shit' (or well, 'Scheiße'). I had slight troubles with the 'sh' sound back then, so may mother sat me down and told me that I'm not allowed to use that word until I could pronounce it correctly.

And that's how I learned to properly distinguish between 'sh' and 's'.
The fucking end!