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.