Books.
There was a book about how they got the Spartans, how they were trained.
they literally STOLE children from thier Families and replaced them with clones that will develop a deadly illness in a matter of... I think it was weeks... But they STOLE children. John, or Spartan 117 as he was called in boot, was one very promising canidate. The woman behind it was the woman who made Cortana too. Cortana is the most high tech AI in the entire human space. I think it was at the age of 17 or 18 they were put into a coma and augmented them. Coating bones with a hard metal, enhancing physical attridutes, enhancing minds. A lot of them died during this process and many more were deformed. Those who survived but were unable to continue in the project went into Inteligence programs. the body was wasted but t he mind was strong. John, Kelly, and if i remember right... Sam? survived. Kelly and Sam? where together during original training. they actually studied using the ideal from the Actual Spartans from the past. Many spartans, after they got thier suits, died when the Covenant destroyed Reach. However many others survived by going underground. the spartans got seperated by many events and the only one still ACTUALLY with the UNSC was Spartan 117..... So..
Wait.. Technically he is right. In the eyes of the UNSC Master Chief John Spartan 117 is the last Spartan.