This desire to do something big is often accompanied by frustration, as we deal with day-to-day bureaucracy, office politics, corporate ladders, academic rigors, or having to jump through hoops in our professional lives. Or maybe we've just chosen a career/job that is far from what we enjoy. We often have big dreams, but feel especially stifled in our early careers.
As we progress in our lives and careers, something happens... maybe our priorities shift, or we realize it's a lot harder (or not worth the martyrdom?) to have the kind of impact we originally thought. Perhaps it was our youthful naivete that created such grandiose dreams in the first place.
A friend of mine once made the comment, "As we get older, we trade our big dreams for smaller dreams." He pointed to smaller dreams like getting another degree, making partner at a firm, owning a house, raising a family, etc. This annoyed me. I don't mean to knock these in any way (they take hard work and are ambitious goals), but it suggests that we are sacrificing our true, high-impact, dreams for the garden variety.
I recognize that priorities change, especially in our late 20s and early 30s, and would never suggest that someone foolishly follow an unrealistic dream. It's important to assess the scale and scope of our dreams with a clear perspective of ourselves and our place in this world, but accepting these smaller "prepackaged" dreams seem like a covert way of justifying "settling" (the nemesis of overachievers).
I say follow your dream, or more correctly, go in the direction of your dream.
Maybe for a 40 year-old IT guy, with a fear of public speaking, the big dream of becoming President of the US is unrealistic (although stranger things have happened!); but instead of giving up on politics altogether, he can still have a profound impact on politics.
Or perhaps a mother of 3 has always wanted to run an orphanage in Latin America. Certainly this is more difficult to do with a family of her own, but she can still affect change in meaningful ways (fundraising domestically for a few orphanages, working at a non-profit with a similar mission, volunteering her business consulting expertise to cut costs at a particular orphanage).
We must remember to hold on to our dreams because they point to our passion and sense of purpose. At this point in our careers, we may be far removed from our dream. My suggestion is to get involved and start small - begin to do it on the side, figure out how to use existing strengths to have a meaningful impact, develop a niche/competency/expertise in appropriate circles, and expand from there.
Live large and shoot for the moon (and then, according to the saying, even if we miss, we'll land among the stars, right?). Happy dreaming (and doing)!