less young than

I have a question. Have a look at the four sentences below.

[1] Mary is older than Beth.
[2] Beth is younger than Mary.
[3] Beth is less old than Mary.
[4] Mary is less young than Beth.

I think they all say the same thing at least logically.
#3 may sound a bit weird, but I think it's acceptable in certain contexts.
How about #4? Is it wrong?
If so, I am wondering why #3 is acceptable but #4 is not.
Could you give me any reason to it?

Thank you in advance.

To my knowledge, nobody ever says "less young" or "less old", any more than they say "more young" or "more old".

It just doesn't happen.

It's "older" or "younger".

I agree. Neither [3] nor [4] would ever be used. I cannot think of any context where either of these would be acceptable. You might, however, say the following.

[5] Beth is not as old than Mary.
[6] Mary is not as young than Beth.

I would put "as" where you have "than", myself. Dialectical difference?

I would also put "as" there. I didn't actually know that anybody used "than" in that context.

No, no dialect difference at all. Just a dull little typo.

[7] Beth is not as old as Mary.
[8] Mary is not as young as Beth.

Oh, okay. ;-)

Can I write IPA with my new Explorer 7:

ɣ ɮ ʁ ʃ

Damn! I was hoping it was a crazy Downunder thing!