Honestly? You do whatever suits your story. I personally have leaving the Red Room at 21, working has a contract-killer and information-thief for several years, the SHIELD stopped her, a year later - aged 28 - she joins SHIELD. In Avengers, she's 34, 35, depending on when you place canon.

I personally can't see her being a teenager and working for SHIELD, but that's my own reading of how they operate.

But, seriously, you don't have to follow anything except the movie, and what's on screen can be viewed differently depending how you want to spin it.