So, there are theories of Phantom Pain being set between MG1 and MG2, with doctor Kio_Marv rebuilding Big Bossess broken body after his epic fight against Solid Snake...
Chances for that are very slim. Just look at Gouda Jouji's timeline:
Ground zeroes start 0-5 years after Peace Walker and in it BB is older, but not much older. MG1 happened 20 years after Peace Walker, there is no way PP BB is 15-20 years older than BB shown in GZ demo.
IMO, PP has to happen well before MG1.