Cara Davis' article, "In Pursuit of Your Passion", offers some interesting thoughts on the pursuit of dreams.
I have heard others (e.g. Aaron Stern, pastor of theMILL--the college/20-something ministry at New Life Church) echo the notion that many (but not necessarily all) can feel free to try a variety of jobs and pursuits in their twenties. God can definitely work through such a journey of exploration and transition. Yet, with that, it has also been said that most should have a pretty good idea of where their lives are going once they reach their thirties. This seems like sound advice in a general sense. Again, not everyone may have that freedom; especially those with families to provide for. Though, as one article commenter wrote: "I think you have to be able to get past the question, "how will I support myself/family" (Ben DeWitt).
Nonetheless, perhaps the nature of American culture today almost even dictates that 20-somethings try a number of different jobs. Many job opportunities (especially in this economy) are short-term, which requires the 20-something to move on and try something else. Also, many job domains seem closely guarded by the older generations (i.e. Builder and Boomers). This has been said to still be affecting generation Y, let alone the millennials (i.e. generation X, c. 18-32 years old). I cannot say for certain in that regard, but I have observed and heard about it on various occasions.
Overall, it seems that there is really no one answer to the vocation question, though many (especially the older generations) would attest otherwise. Each person has a unique journey to pursue. That seems to be God's way. He is above any preconceived set of rules or formulas. Not that such things, which have been found to be generally true by our predecessors, should be cast aside. Rather, the important perspective is that such rules are guidelines at best. Guidelines, not laws. This reality then requires abundant grace and patience. It requires discernment of the Holy Spirit, and it requires perseverance. But, above all perhaps, it requires faith in YHWH. He is truly greater--His scope of power vaster--than anything we have ever perceived and ever will perceive. To God be all the glory, forever and ever. AMEN.