Solid Snakes rank in the US Army before joining Foxhound.

Alright so here's some real Army math for y'all.

Being born in 1972 Snake would have been 23 when he joined Foxhound and was deployed for Operation Intrude N313. it's impossible for Snake to have been an officer in the US Army because he would have had to go through college and ROTC to commission and the timeline doesn't account for that so that's out.

In reality to join SF you need to have 18 months in service and the rank of PFC. So for the sake of argument let's say Snake was given a waiver to enlist at 16 years old. He then would have gone through OSUT which is infantry Basic and spent at least a year on site at whatever duty station he was posted up at (with 1 possible deployment probably during the Gulf War). At 17.5 (ish) years old he would be eligible to go SF and begin his 5 years with the Green Berets and upon completion of that training he should have made the rank of Sergeant by age 19.

From here, if granted a waiver for promotion, he would have be able to make Staff Sergeant by age 20.5. At this point we know he served until he was 23 before joining Foxhound and being the super soldier he is it's safe to assume that Snake left the Green Berets as a Sergeant First Class. So there you have it, SFC Solid Snake.

Edit: As pointed out by u/MasterGoshinki, it is required to have 7 years in service before promotion to SFC so I stand corrected as it's more likely he would have been Staff Sergeant Solid Snake.

Edit 2: Taking in to consideration the changes in military training and time in service/grade requirements over the past 27 years we can't paint an exact picture of the process that got Snake to where he was in the Green Berets before joining Foxhound but my description above uses modern standards from what I know through experience and talking with SF recruiters. that said I think the safest bet is to probably go with Staff Sergeant as a general assumption of Snakes rank at that time.