ANSWER: It is good that you are reflecting on being baptized just before you became a teenager. It may be that God is pricking your conscience and wants you to renew your relationship with him. It is unfortunate that, unlike you, most people in this world do not want to have a real relationship with their heavenly Father.
There are logically only two possible outcomes that could have occurred when you were initially baptized at the age of 12. The first is God gave you the gift of his Holy Spirit after you rose out of the baptismal waters. The second is you did not receive God's Spirit and all you ultimately got from the ceremony is becoming wet. If you wish to delve further into the steps that constitute a Biblically proper baptism please see our article on what this ceremony symbolizes.
The only justification for being baptized yet again is if you believe you did NOT receive God's spirit when you were young. If you did receive the Holy Spirit some fifteen years ago, then performing the ceremony again seems a bit redundant and unnecessary.
As you are probably aware, the decision to commit your life to God and become a Christian is the MOST important one you will make - EVER. It is far more important than deciding whom to marry. While mates can come and go (sadly, due to the hardness of human nature) God will love, guide and stay with you NO MATTER WHAT unless you consciously and fully reject him and his ways.
Below are some things to consider regarding the period before your initial baptism. If you are willing to honestly answer the below questions to yourself, then your ultimate decision will become much clearly to you.
Did you consciously repent of disobeying God and living your life as YOU saw fit rather than according to his will?
Were you aware at the time that becoming a Christian was a lifelong commitment that would require constant work to CHANGE, until the day you died, how you lived and thought so that it conformed to what God wanted?
Did you fully accept Jesus Christ as your personal Savior and understand that it was his sacrifice that made your calling possible?
Was the decision to be baptized, even in part, due to pressure from others (parents, church, friends, etc.) or to be accepted by any one or group of people?
Do you feel your emotions at the time may have swept you along to make your decision?
When everything was said and done, did you feel that the right thing to do, the thing your whole life seemed to prepare you for, and the only decision you could possibly make was to become a Christian?
In the final analysis, the decision whether to be baptized again or not it yours to make. You should resist any pressure someone may try to place on you to decide one way or another. No one should make this pivotal decision "for" you. While others may help you decide by providing support, pointing out things that need to be considered, giving feedback on how they honestly see you living your life, and so on, it all comes down to you and God. After all, we are talking about a decision that will affect your life FOREVER.
I suggest praying to God and talking to him about being baptized again, asking him to somehow confirm (if you need it) whether or not you have his spirit in you. Our loving Father knows when it is best to call a person to repent, change their ways, and follow him. He alone knows when the time is right to give someone his Spirit. Laying all your thoughts and concerns before him, asking humbly for help in your decision, is possibly the greatest thing you (and anyone else for that matter) can do in this age.