
I'm an American English speaker, and I sometimes say "not so ... as ...", but I'll admit that it's definitely less common than "not as ... as ...".

There is a difference in meaning between "not so young" and "not as young"
Example of correct usage for "not so young":
1. However, I was not so young as not to take the precaution of carrying with me a faithful servant..
2. She was not so young that she couldn't tell right from wrong.
But "as young as" is just a comparison.
Of course when I googled "not so young" I found a lot of erroneous usage. I"m wondering whether 'not so young" is used in places which historically had a lot of German immigration, because it's just like the German "nicht so jung".
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