If we take the journal as canon Ford writes some stuff in there that can help us narrow things down.
Two calendar years pass over the course of its writing: it starts in the summer, covers another summer, and then the portal test happens in January. This means Ford started writing it in 1980.
During Ford's encounters with the ghosts he says he's in his thirties. The ghost pages aren’t dated but take place some time between the two summers, so this is either ‘80 or ‘81. The youngest he could be is 30 in ‘81 and not yet 31 when he went through the portal in early ‘82, making him and Stan at least 61 in the show.
This is also consistent with the scene where Stan gets kicked out, because Stan and Ford's principal had a picture of Richard Nixon in his office, who started his presidency in 1969. If the twins were 18 in 1969 that would mean they were born in 1951, once again making them 61 in 2012.