4 Best Theories On How Coulson Is Still Alive
#1 He's a Super Soldier Like Cap!
Coulson is Cap's biggest fan because he always wanted to be like him. What if the "vintage" set of Captain America cards Coulson collected is his from when he was a boy during WWII?
Coulson's character fits all of the requirements that Dr. Abraham Erskine (Stanley Tucci) explained to Rogers of why he had chosen him, so what if Coulson got some other version of it later, after Cap disappeared? The military still wanted a super soldier serum.
Fun fact: The kid in Captain America: The First Avenger who sports the painted garbage can Cap shield is named Phil. The serum explains the slower aging.