Kyosuke_Uchiha
I choose "later" options. My Reason: 1. Need some backers or connections to build all of his needs before leaving (vibranium, adamantium, technology, weaponry (guns and close combats weapon), armory, information, free passes to go anywhere, etc). 2. Making the contracts with SHIELD as side job with emergency call when the main event begins, after all his power already surpassing the avenger team battles. 3. After leaving with the materials he got, he can create A.I for anti hacking, fighting JARVIS, hacking all organization that is known in Earth, manipulating and playing bussines stocks and managing his business company.
Depends on what is the goal of MC. If he just wants to be a hero, then he can keep in touch with SHIELD and do whatever he wants. If he wants to build a company like the STARK, then he can be like Tony. Be a part of SHIELD without being in it. If he wants to travel the Universe, then he can talk with Thor. Moreover, he is just 10-12 year old... He needs to go to highschool.