At least 72, because that's when Birdseye offered the first frozen food to the public. (1930) The argument for 64 would be good, Nescafe, instant coffee was offered in 1938.

But then Grandpa couldn't have a legal drink because prohibition was from 1920 until 1933.

The point is well made, and even I can remember five and dime stores, when a nickle candy bar, was only a nickle. ;-)

How about this, could Grandpa legally pick up a casino chip in Las Vegas? That would help pinpoint the date. lol