Jason, Jennifer, Thomas, Karen and Kevin were all typical teens.
That is, if you consider typical to be having the ability to fly, throw cars around like toys, and control machines with your mind.
Three of them, Jennifer, Thomas and Karen, had been friends since elementary school. When they started developing their powers, they had their friends there to help them through it. Their fourth grade year Jason moved into their area. He was in fifth grade, but he found the three of them and joined their little click. In middle school they met Jennifer, and that was that.
Once in high school, they took as many classes together as they could. With his power over machines, Thomas hacked into the school computers to make sure they had all the most number of classes together as possible. So when FBI Agent Ralph Knox found them they were all in class together. In fear, they panicked and ran, and as soon as the bell rang, disappeared into the throng of students.
But after he proved his intentions, they joined with him. Names were tossed around for what their superhero group would be called.
Kevin voted for Men in Spandex.
Thomas, his new superhero name being Virus, wanted them to be called a Blight on the hindquarters of Crime.
So in the end, they just became the five superhero teens, shortened to the Five.
At first Knox gave them weapons and costumes. Later, they found that it wasn't enough.
So Knox delved into his past. He returned to the alien technology that he had been in charge of during his earlier time in the FBI. And he gave it to the Five.
So when Snow Leapard finally joined, Knox was ready with new costumes and technology for her.