ODSTS have existed for as long as the UNSC has existed. Anyone and everyone can join. There's technically no age, weight, or experience regulations. If you're a good soldier, you get taken in. ODSTs are trained in the state of the art squad tactics. Possibly on par with SPARTAN squad training. ODSTs are generally deployed via SOEIVs that are maneuverable in flight. Not everyone is guaranteed to survive. All SOEIV are equipped with extra fuel in case an ODST needs to reposition him/herself.