I'm 20. Can I still grow taller?
The answer is probably No.

"Long" bones - like the tibia, fibula, femur, humerus, etc. grow at the ends during childhood via a special formation called the Epiphyseal plate (also known as "growth plates"). The plates are composed of a special cartilage that grows, and slowly calcifies as a person reaches adulthood. 

When the growth plates completely stop expanding is left to genetics, but generally happens 2 years after puberty finishes - about 16 for women and 18 for men. So after those ages there's little, if any, significant growth - though it can happen in very rare cases.

To date there aren't any exercises you can perform that will elongate bones. It doesn't make physiological sense; stretching puts stress on muscles and tendons - not bones. Weight lifting will compress bones, making them more dense and will make you shorter in the long run.

While there's been some success in getting chondrocytes to grow cartilage in vitro, there has yet to be significant gains in vivo, thus there aren't any medications available that would cause growth.

The only widely practiced way of gaining height after adulthood is with very painful surgery that involves a lot of braces. The bones are systematically broken, held in place, and allowed to heal. The setup looks like this:



While we get closer and closer to being able to control such things every day, for the time being (and I wouldn't count on this changing anytime soon) everybody is stuck with the height they were genetically programmed with.

As an aside: Being tall isn't all it's cracked up to be. At ~197cm I can tell you it is hell finding pants that fit.