At the cemetery, the headstones of the greasers say they were born in 1946 and died in 1963, that would make them 16-17 at the time of death. The three actors Robert Rusler, Nicholas Sadler and Bentley Mitchum were all aged in their mid-20s at the time, which is obvious.
After the greasers run Billy off the road, two men fishing from the bridge run over to help him, and they have two tackle boxes beside them. A few seconds later when the greasers drive away over the bridge, only one of the tackle boxes is still there.
The train accident that killed the greasers happened in 1963. The train that hit their 1955 Chevy, killing them, was the Southern Pacific 5021. The Southern Pacific 5021 was retired in 1955 and was the only train of its kind preserved and donated to the Railway and Locomotive Historical Society on March 8, 1956. It's not possible for that train to have killed the gang in 1963.
The principal tells Jim that Lawson is a transfer from Milford, which is what Lawson and Vincent tell Jim when they first appear in class. Later Jim tells old Officer Neil he can't find any Milford High, which Neil tells him isn't a school, it is the cemetery where they reside. Surely the principal would have known there was no school by that name.
When Jim's son Scotty finds the pair of red sneakers, Jim states that he used to have a pair just like it when he was a kid. However in the flashback scene of the young hoods roughing up Jim's brother Wayne, it was Wayne wearing the red sneakers. Jim had different colored footwear.
The headstones of Richard, Vinny, and North show that they all are born in 1946/47 and died in 1963 - making them 16/17 at the time. Wayne's headstone shows that he is born on the 15th of January 1948, which makes him only about 1 year younger than the evil trio. However Wayne looks like he is at least 10 years younger than any of the three kids.