From what I gathered the Spirit Forest shenanigans happened about 20 years ago, and since Korra seems to be about 18 Aang was almost dead or maybe even dead. Even if he reacted as soon as he got word of the forest being destroyed, which may have been late since I imagine he was in Republic City, he wouldn't be able to get to the Northern tribe in time to stop the rampaging spirits since I think it happened the night after the forest got destroyed. I guess he could have reopened the spirit portal or something like Korra did but he was possibly even dead at that point anyway.