Spartan-II's where kidnapped at 6 Spartan-III's varied in age as they recruited orphans from glassed colonies and offered them a chance at revenge against the covies. Sartan-IV's are from all other military branches that pass the requirements test get some augments and armour. A fair few are ex-ODST's As for the Spartan-1's they where regular soldiers recruited into the Orion project and tested on to try and create super soldiers not sure how many there where or how successful they where but Sgt.Johnson was from the S-1 programme.