system
Given the following post, respond as if you're commenting on Reddit:
human
[S.H.I.E.L.D.] Which real life intelligence agency does SHIELD most resemble? Is their a minimum age or education requirement to join SHIELD? How long does it take to train in SHIELD?
gpt
In real life, the CIA is the closest parallel to SHIELD. They're both espionage and intelligence agencies with a history of covert paramilitary action. The biggest differences is that while SHIELD was founded by the US Government, it became an arm of United Nations with a great degree of independence from the US. 

>Is their a minimum age or education requirement to join SHIELD?

Are you valuable to SHIELD is all that matters. Most agents either have a background in the military, espionage or superhuman abilities.


>How long does it take to train in SHIELD?

It's not standardized. Quake (Daisy Johnson) reached level 10 (the highest rank possible for a SHIELD Agent) at just 17 years old.