The cutoff age is generally taken to be around puberty, so at 22 I kind of missed the boat. The only area of language acquisition where this really holds, though, is pronunciation. People who learn a second language after puberty generally have a foreign accent their whole lives, while those learning it before puberty will have a native accent.

Built-in cutoff switch?

One linguistics professor of mine had an interesting hypothesis as to why this was so. As we know, our use of language identifies us culturally, socially, and regionally. He reasoned that when we're children, our job is to assimilate into the surrounding culture(s); but when we're adults, our job is to be productive members of the community, and identify ourselves as members of it. He thought this was why the ability to pick up foreign accents "shuts off" after puberty. At this point we're supposed to be identifying ourselves as members of our group, not haring off to join those guys over in the next valley.

I should also note that some rare people learn a second language as adults, and don't have a notable foreign accent. But they will tend to make very occasional slips, which become more frequent when tired or stressed. There have also been some inconclusive studies suggesting that adult second-language learners process their second language in a different part of the brain than early language learners.

But aside from pronunciation, adults actually learn the grammar (syntax and morphology) and usage (semantics and pragmatics) of the language just as quickly as children, given the same amount of stimulus. That, of course, is the problem: adults tend to have all sorts of responsibilities that prevent them from devoting as much time to language acquisition as children.

Another problem is "fossilization" — when the learner stops making progress in the second language. This is more common in adults, whose brains aren't as flexible and who don't pick up new ways of thinking as readily as children.

How hard is it?

Obviously, it's possible to become a translator even after learning the B language as an adult. I'm one example. Since we don't use the spoken language directly in our jobs (although it helps when trying to get work from foreign clients), pronunciation really doesn't come into play.

My pronunciation of Japanese actually isn't that bad. I suspect this is partly thanks to my degree in linguistics, which helped me learn to analyze and mimic the accent of Japanese. But my accent hasn't really come into play professionally. Knowledge of spoken Japanese, yes — but not my own pronunciation of it.

I'd say it takes two main qualities to learn the B language well enough to translate it as an adult:

1. An open mind and a desire to continually learn
2. The perseverance and dedication to spend a lot of time mastering it

Assuming you've got the first one, what it really boils down to is perseverance: being willing and able to put in the time necessary to learn the B language well, when all those pesky adult responsibilities like earning your rent money are intruding on your time.

How much time do you have to spend? Various studies seem to show that no matter what the field, it takes about 10,000 hours to master it. This would mean using the B language 20 hours a week for roughly ten years, or 40 hours a week for roughly five years. That's a lot of time to put in!

And then, of course, you've got to master the skill of translation — pile on another five years of working full time — plus another five years full-time to learn a specialty! Fifteen years to become an expert translator? Might as well give up at this point and do something easy, like become a cardiologist. They have worse hours but better social status and make more money.

So add two more necessary qualities: a deep love of the craft of translation, and a certain stubborn streak.

Conclusion

It's difficult but not impossible to become a professional translator after learning your "B" language as an adult. What it requires is perseverance, an open mind, and a constant desire to keep learning.