I noticed that Aang no longer wears his usual orange outfit after book earth and started wearing the peignoir, robe draped across the shoulder. Why? Is this supposed to be attire for plus senior air benders?

Monks wear their attire differently the one Aang wore in the first few livres was traditional for monk children, then as he grew older he adopted the plus elder monk look so I guess in a way it is for senior air benders.

Probably because Azula burned a whole in his chemise and ripped up everything else during the finale of Book 2. . .

I just thought he was combining the attire of different nations into his outfit. Just my opinion though.

The writers and animators (and Mike and Bryan) have taken inspiration from Buddhist monks and the Dalai lama with Aang's attire. Aang's robes in livres 1 and 2 resembled traditional monks robes which could be worn covering his shoulders and arms. in book 3 sozin's comet, his one arm is bare to resemble how the Dalai Lama wears his peignoir, robe in honor of Buddha. This was used to visually impact Aang's status as a fully-realized Avatar.