Its hard to say without being there to hear the interview.
1. Are you neat & professionally dressed?
2. Are you on time....10 mins early?
3. Do you follow up after the interview?
4. Have you had any problems where you worked as a CNA/intern?
The experience as a CNA and intern should help you, and your age should not make a difference....unless you come across as immature. Practice interviews with a friend or someone that does hiring for any occupation.....get their feedback. Also it could be that the jobs you have interview for could have gone to someone with a lot of experience. Good luck and I hope this helps!