I have heard this type of sentence with the same structure every time: 'Beautiful as she is', 'Intelligent as he is...'
Is it grammatically correct and if it is what's this grammatical phenomenon called? How do you end such a sentence? I believe it's Inversion but I'm not sure.
For example with a sentence beginning like this 'Young as she is...' how would I end it?
These are not sentences but heavily reduced versions of much fuller clauses of the form
[X SUBJ BE as] ADJ as SUBJ BE
[X she is as] young as she is
The construction SUBJ BE as ADJ as SUBJ BE may be paraphrased
SUBJ BE so ADJ
she is so young
"X" here is a term of the sort traditional grammars call subordinating conjunctions and contemporary grammars tend to call prepositions. X is usually understood to have the sense of either because or although.
Such clauses are typically employed as clausal modifiers with either a consequential (because) or a contrastive/concessive (although) sense. Which sense is intended is usually clear from the context:
CONSEQUENTIAL: [Because she is as] Young as she is, she doesn't have the maturity for this position.
CONCESSIVE: [Although she is as] Young as she is, she's too old to play this part.
You will often encounter this sort of clause with the first as still in place:
As young as she is, she's too old to play this part.
I wouldn't be able to tell you how it's called, but it is grammatically correct, and conveys a contradiction between the first and second propositions. A simple example, since you asked for it:
Young as she is, she's already accomplished many things.
Despite the fact that she's only lived for so long, she's already made those years matter. This is equivalent to:
Even though she is Young, she's already accomplished many things.
As TRomano pointed out, talking about concession is even more accurate.
