Complex sentence: ATAC is a top secret government agency co-founded by Fenton Hardy, he created it when he realized that there are some places where an adult spy (a FBI or CIA agent) would stand out too much.
Split into simpler sentences: ATAC is a top secret government agency co-founded by Fenton Hardy. He created it when he realized that there are some places where an adult spy (a FBI or CIA agent) would stand out too much.