I'm 23 yo, learning japanese for ~4 years already, but until recently with little success.
That's it, until april/17th/2008, when I found out about a blog called "All japanese all the time". It gave me a boost of confidence and everything changed.
In april/18th/2008 I started to go trought Mr Heisig's Remembering the Kanji book, as Kaztumoto sugested. In july/19th/2008, I finished it. Only 3 months.
I skipped the kanas, wich I already knew.
Since july/27/2008 I'm doing sentences.
I notice improvement in my japanese daily.