I also strongly recommend Chris Whitcomb's book. It's a fairly unflinching look at life in the FBI and on HRT. It explains a bit about how things got so screwed up at Ruby Ridge and Waco. It doesn't absolve those truly responsible for what happened, nor does it absolve Lon Horuchi for his killing of Randy Weaver's wife, but it provides a good snapshot of the mindset of the HRT operator, the conditions they work under, the selection process, training, esprit, etc.
Whitcomb is still an active agent with the FBI so he can't make any outright statements or attributions of blame for what happened...but there's a lot written in between the lines.
The minimum age for acceptance as a Special Agent is 23. There is a requirement for a minimum of three years working experience in the real world. Military experience may be helpful, but advanced degrees in law, accounting and business are strong recommendations as well.
the background check the FBI puts you through is exhaustive. Thanks to Clinton's administration, the casual drug use disquals were loosened a touch, but if you were ever arrested for such things, your done. Lying or trying to hide info from the FBI is a thorough waste of time and energy...they will dig it up and deception is pretty much an automatic disqualifier.
Psychological make up is also important. The FBI wants stable, confident individuals with patience and the ability to work well under pressure. Teamwork is critical. Obviously those are ideals and some odd birds slip through.
I wish I had given it a try when I was in the age range, but at that time I lacked the confidence I would have needed anyhow. Give it a try.
As far as the guys who turned Ruby Ridge and Waco into such disasters are concerned, politics always interfere with any organization larger than about 5 people. In large orgs where significant power is exerted, the politics get pretty bad, be it a corporation or a government agency. You can't avoid it. All you can do is work to do the job right and avoid the political entanglements. 95% of the time, your operations will go off properly without any politically induced tragedies. Most of your tragedies will be the normal type resulting from unforseen circumstances, dumb luck, innocent mistakes, etc.
Good luck.