As of one of the 0.12.1 builds tilling grass no longer can yield seeds. On old type worlds there is no naturally generated grass (not grass blocks). With out these it is hard to get seeds. The only way to do so is to kill a skeleton and hope to get some bones and make bone meal and grow your own grass that you can hope to get seeds out of.

I think grass should naturally grow on old type worlds so you can get seeds easier on old type worlds.