You have to be ex military, usually special forces to join black water. Most blackwater personnel are in the 30's-50's.
The minimum age to join the US military is 18, after which you undergo basic training then advanced training, so by 20 you have only 1-2 years of experience.
You really think, a 20 year old is capable of being a special operative.
Think back of when you were 20 and how mature you were.