I'm only replying to the second question. In the US at least, it seems (and at one point I might have been able to point to the studies) that interest in STEM drops off[1] for female students around middle school, same for minority students. Many efforts at outreach, then, have tried to focus on introducing extracurricular academic programs at those students (often by targetting schools with higher female or minority populations) in an effort to cultivate their interest and maintain it through high school, in the hopes that it can increase the number participating in STEM in college and, later, the workforce.

At this point, however, I don't know what measurable impact those programs have had, I should probably talk to the folks I know that are still participating in the programs at a level where they monitor the trends.

Regarding who: Anyone working in STEM with the patience to deal with 12 year olds. Volunteer with FIRST Robotics teams or other after school or summer programs. Female and minority volunteers, in particular, as knowing that there are female and minority STEM workers isn't the same as knowing female and minority STEM workers, it can make a huge difference for kids.

[1] EDIT: It may not be a drop off. It's been too long since I was involved with this, but perhaps instead of them losing interest it's that they don't develop the interest at this point?