When you enter the section for dependents and type in the information about your children the system will ask for their birthdates. Once you have indicated that they are over 17 the system will ask if they are full time students. Answer yes to this question and you should be all set.
Once you have indicated that they are over 18 (and under 24) the system will ask if they are full time students. Answer yes to this question.
For students over 23, later, in the educational expenses section, you will be asked to enter their student info. Students, over 23, do not qualify, as your dependents, if their income is more than $5050.