I've Been on Zoloft (sertraline) since january, & I'm 20. It says a side effect is sexual problems even after you stop the med. How likely is that to happen to me?

If you don't. have such a problem with libido while on Zoloft (sertraline) it is unlikely you would develop this upon discontinuation. Be sure to let your doctor know about any symptoms or medication changes you are making.
Not very. One of the best ways to reduce the chances is to come off the Zoloft (sertraline) as soon as is clinically appropriate - the tapering is important and if done well, after-effects are less likely. Overlap the tapering w/ a CBT or OCD experienced psychotherapist and it will also make for a softer landing. Expectations for sexual problems make them 100% more likely - so don't. You should be fine. Best wishes!