When an emergency occurs your training will kick in. Follow your NRP and CPR protocols, call for help and people will come running. Most NICUs have double checks and policies and procedures in place to prevent errors, as long as you follow these you should be okay. You will get extra training on caring for neonates, and with a little time you will learn what to expect for orders. If you are ever confused or concerned, coworkers should be around to help.
It doesn't really matter what level a NICU you start in, you will start off caring for healthier babies and slowly be worked into caring for sicker babies. IMO It can be helpful to be in a higher level NICU, because you will see more and learn from others. Starting off in another area of nursing isn't a bad thing, it can help you gain experience in talking to docs and families. I felt comfortable in about a year, 2 years for really sick babies.
I am turning 28 in 2 days, and frequently get ID'd, families at work take me seriously because I speak confidentiality, am knowledgeable and professional. When I was newer, I never wore colorful scrubs as I figured they made me look younger. Now I think it's all about how you present yourself. Introduce yourself as a nurse and they will have no reason to doubt you.