So I'm not exactly old but I'm not exactly "Tumblr age" (13-17) either.
However there is a really awesome Tumblr I follow focused around Mark
Twain.  There is a heavy influence of Anime on this Tumblr (which I find
rather tasteless), due to the age range of this following growing up
alongside an Anime adaptation of Tom Sawyer and Huck Finn.

If you are curious how Mark Twain is being absorbed by today's youth, take
a visit.  Overall, it's a delight:

http://huckyeahmarktwain.tumblr.com/