Is abs workout okay for my age (18)?

Of course: Exercising the abdominals is part of a good routine of healthy exercise. Hopefully you have a program with the advice of a physiatrist or exercise physiologist. The abds are also a good portion of ones self image.

By all means: You're at your physical peak and should cherish this time. For the best definition, eat enough protein and beyond this, only to keep hunger at bay, and resolve to be hungry much of the day. Here's a father showing his 13 year old son how to do basic sit-ups, but you have many other possibilities -- cable pulls are great. I'm proud of you just for asking. Best wishes.