There are four criteria for a child to be a qualifying child, they are:
1-Relationship
2-Residence
3-Age
4-Support/Income

you are asking about the third criterion which is Age. For Age there are two limits for the non disabled child. The permanent and totally disabled child can always be claimed as a dependent (provided he meet the other tests).

for the Age test your children need to meet one of the following:

They must be under age 19 at the end of the year, or under age 24 and be a full-time student. Full time student has its own meaning for IRS.

I hope I have answered your question, if I can be of any help please let me know.

Regards,
Abraham Itani, CPA