Amateur players do not "declare for the draft." Not in baseball.
You need to read Rule 4. The Major League Rules are here (pdf). See pages 47 and 48.
Summary: All US players who are eligible (if they've graduated from high school, and and aren't matriculated in a 4-year college - or if they've been so matriculated for three years) are subject to the draft. No "declaration" is required or possible under the rules. In fact, there is a procedure for an eligible player to have himself (herself?) excluded from the draft in a given year - it requires written notification to the Commissioner's Office. Failing such notification, anybody who meets the the eligibility rules is "subject to the draft."
Put another way - I am subject to the draft. andyb is subject to the draft. LoneStarPhan is subject to the draft. California Dreamin'... is not, I don't believe, subject to the draft, because of past associations.
Now, I'm not going to get drafted - but my eligibility, which began in June 1972, only to lapse again a couple of months later, and subsequently re-started - has never ended. I'll be subject to the draft until I die, or until I sign as an undrafted free agent. (I'm waiting...sittin' by the phone. )
Enough of the rant. Just remember, MLB is unique. Kids don't have to ask to be drafted; they have to ask to be excluded. That's why, every year, some clubs draft high school players who have made it clear they're not going to sign, period, but are going to college. Clubs define them as "tough signs," and draft them in later rounds. Occasionally, somebody changes his mind; often they don't. But those kids didn't "declare" for the draft, and then turn around and say, "I said I want you to draft me, but I'm not going to sign!"
I rant about this because I don't like it much. It's a restraint of trade - and you can't argue that the kids agreed to it by "declaring" for the draft. They haven't... and you can't argue (well, you can, but any self-respecting judge would throw you out of her courtroom) that, by not bothering to formally demand exclusion from the draft, they have "opted in."
Occasionally... not often enough, IMHO, to drive the point home, but occasionally... a kid is drafted who decides he'd rather do something else with his life. I don't know - go to medical school, perhaps. Doesn't happen much, because generally, kids who get drafted are baseball obsessives, but it does happen - because the outlier who really wants to be a pediatrician or something gets drafted even though he really didn't want to be a professional baseball player. That would not happen if players had to "opt in" to the draft... but more kids might actually thing about life options before they were staring a signing bonus in the face.