My father was a teenager when he, my grandparents, and his siblings began learning about the Church of Jesus Christ of Latter-day Saints; after he and his family had been studying with the missionaries for a while, he prayed to know for himself. I have heard him describe his experience. For him, it is a clear moment in time where he transitioned from not knowing to having a personal witness of the truth, a moment he described as "as sweet as the juiciest orange" and a feeling that "filled his entire body."
As a teenager, I sought to have the same pivotal event, but that is not how I received my answer. As I prayed, the subtle thought came into my mind: "do you not already know?" It was not what I expected, and it did not come with an outpouring of the Holy Ghost that I could discern at that time. In the moment, I'll admit to being a bit disappointed, but able to answer that yes, I did know. At that point in my life I had already received many answers to my prayers, felt enlightened and uplifted in many ways as I read the scriptures or and tried to live the Gospel, felt the peace that comes with temple attendance, and many, many times felt the Holy Ghost confirm to me that the principles my parents taught me where true.
However, gradual changes are harder to notice than dramatic ones. When I was younger, sometimes I wondered if I really knew, or if all the little experiences could be explained away somehow, and I was fooling myself. I have since learned that many have similar experiences. We find this counsel in the Doctrine and Covenants:
As I have continued to gain experience attempting to live the Gospel, I have come to understand many things about the answer I received as a teenager. Mostly, it has helped me pay attention to the myriad confirmations I have had since of the truth of the Gospel and of His church; some have been simple and subtle, and others have been profound, clear, and accompanied by an outpouring of the Spirit.22 Verily, verily, I say unto you, if you desire a further witness, cast your mind upon the night that you cried unto me in your heart, that you might aknow concerning the truth of these things.
I believe I have benefited more by the answer I received than if I had gotten the answer I originally sought as a young man because it lead me to examine my life. That by itself is a great lesson as we seek our testimonies: answers will be individual, because God knows us and loves us individually. We cannot dictate to Him how to do it, but it is my experience that if we will seek Him then He will answer and will show us of His love.