Its been confusing me for a while, like its got music from the 60's and 70s so I guessed early 70s but then some of the clothing seemed a little off (I could be wrong tho) and at the end they were old men and if it was set in the 60's or 70's they would only be about 50 now wouldn't they? Did I miss something where they said what year it was?
EDIT: wait never mind I just found another post about it
In the book the story was set in the 30s, but the filmmakers decided to switch it to 1969-1970. For proof, "Spirit in the Sky" plays on the record player in the cave and that song was released in 1969. "Have You ever Seen the Rain" plays on the soundtrack later and it came out in 1970. Some people found this confusing, but I didn’t. They were in very remote locations so it makes sense that there were archaic vehicles from the 40s or whatever mixed with more modern ones, like the motorcycle. In one scene a dilapidated late-50's automobile is shown rusting away in the desert.
Others complained that the children-as-adults at the end were too old considering they were kids in 1969/1970, but the epilogue obviously takes place in the near future (in reference to the time of the movie's release, 2007), which would make it sometime in the 2010s or 2020s. This would put the characters well into their 50s or 60s and the actors looked it.