That's so interesting. I too got way more invested in HP after POA, which is where we're introduced to the Marauders -- both the school age ones and their sad, damaged remnants. Granted they were the main characters, but the kids never held my interest; not fandom-wise, anyway. It was the adults in the background, the ones who had terrible choices or been bruised by life, that fascinated me.

Tumblr can be a lot of fun, but the average age there is much lower than in HP fandom and sometimes, boy does it show. I can rec you a blog to check out, though: "cumberbatchitis". All things Sherlock and Benedict. I know the blog owner from another fandom, she's wise and lovely.