Do some research on the topics of discussion for this event. A basic understanding of those topics will help you to better understand what is discussed during the event, and that will allow you to have better discussions with the people there.
Also, if the schedule lists who will be speaking at the event, then do research on their professional backgrounds - what projects, technologies, and products have they have worked on? Research those things to gain enough of an understanding about them so that when you approach those people at the event, then you have some topics to discuss.
During the event, pay attention to what the speakers say. It may be a good idea to take notes so that you can remember important points and also things that you'd like to ask questions about. Then during the "networking" portion, you can approach the speakers and mention what you found interesting about what they said, how it applies to the company/field you are in, and ask them any questions that you may have. You can also ask about what you learned while researching their professional backgrounds. For example you could ask what technologies they used on a previous project that you find interesting, what skills/experience a person should have if he/she wants to join their company/field, etc. These questions will show that you took initiative prior to the event to do research so you could come to the event prepared, and they will probably be impressed by it.
Then after the event, go on LinkedIn (I suggest you create a LinkedIn profile prior to the event) and try to find everyone you talked to and send connection requests to them. Then every once in a while, do something on LinkedIn that will appear in their news feed (e.g. write a status update, update your profile, follow a company, like a company's status, join a LinkedIn group, etc). By appearing in their news feed on a regular basis, you ensure that they won't forget who you are, and that will make it easier for you to contact them later for advice or job opportunities.
Finally, you should definitely show up in a suit to give yourself a more professional look since you've had problems with people thinking you are a teenager. In addition, give your business card to the people you talk to, so they know you are a working professional. If your company didn't give you any business cards, you can easily make your own for a low price on the internet. At the same time that you give your business card to someone, ask for theirs so you have their contact information in case they aren't on LinkedIn.
