SEALS enlistment process - Age?

I'm curious about the process to become a seal, and what my deadline is. As I understand it, you enlist in the Navy under DEP with whatever rate. You then request the PST, and if you p, you get to go to BUD/S. Is this correct? Also, I understand you have to be under 28. Do you have to be 28 when you take the PST, start BUD/S, or end BUD/S? Also, how far in advance are BUD/S cless filled?