I'm under 30 (at least for 32 more days I am), too.
It's been tough. I've actually become MORE of a baseball fan as I've gotten older. I grew up in central MO and actually rooted for the Cardinals as a secondary team. Then I moved to St. Louis for work and dealt with the Cards fanbase as a whole for the first time.
My Cardinals fandom shrunk accordingly. Thankfully, that was right around the same time the Royals began to show glimmers of hope again...
"You gotta love livin', cause dying is a pain in the ass."