First, read this answer and ensure you do none of the things on this list. The last thing you want to do is for your behavior to reinforce your youth. Frankly there are things you can do to minimize the appearance your youth has. My guess is you could find many things on the list in the linked answer you could easily improve on and improve your image. Read it and make it part of who you are.
Second, realize no one cares what your skills are - they care about their impression. You could be 20 and a genius or 50 and an idiot, what people you interview will think is their impression and how you make them feel about your skills.
Practice on the impression and image you have. This will make/break many people's confidences in you.
How to handle yourself during the interview
Ok, you created an app of some sort which took off and now you are starting your IT/web empire. So what? Many people have 1-hit wonder projects. Why are you special? Was your success blind luck? How will you continue your success?
This is what you need to communicate to any more senior person. At 20 you don't have a lot of life commitments and a steady salary is not as important as it is to someone who might have a family and kids and mortgage. But you need to realize many if not most people you interview will have those commitments. Even though as a startup you will prescreen many who significantly value stability (they won't apply) everyone else still will want assurance you have a company which will be around for more than a few months.
It sounds like you went from a single person to a company requiring many highly paid positions - DBA, lawyer, Sys Admin (and probably someone to handle the admin side, like a multipurpose HR person or office assistant/etc). You are looking at a monthly budget probably close to $500k if you pay close to market rates for only a few employees.
The necessity of this will be obvious to anyone with experience interviewing. Can your business model sustain this? Anyone interviewing better come away believing the answer is "yes." For that matter, YOU better believe it.
So does age even matter?
An observant reader will note the entire above section applies to almost all startup companies regardless of your age. It doesn't matter if you are 40 or 20, those factors are important.
It might be more important if you are younger as at least older startup founders have a "hey this guy has done this whole business thing for a while" going for them.
Frankly speaking, most people you want to hire will rather work for someone eager to learn and take advantage of the expertise on the team -- regardless of age -- than work for someone who is disrespectful. Keep this in mind (which is why perception/impression are so important).