If who we see in the trailer is Big Boss then this game must take place shortly after MGS Peace Walker. Peace Walker was set in the year 1974 and Liquid and Solid Snake were born in 1972. If this game takes place in 1980, my guess would be that the kid in the beginning of the trailer is Solid Snake. Holy crap

Solid Snake was born in the year 1972, this game shows that the FOX unit is still active, which is the unit before FOX-HOUND. My guess is that, what we see in the trailer is Big Boss and the kid in the jail cell is Solid Snake about 7 years old.