主要还是位置的不同：although表示“尽管”时，比较正式，放在状语从句句首。as表示“尽管”时，结构是adj/adv/n（表语/状语）+主语+系动词/谓语+其它

例

Although she is young, she knows a lot.

=Young as she is, she knows a lot.

as还有很多意思。因为、当……时候等等。