In the book it's mentioned that Ponyboy has recently turned 14 (S.E. Hinton gave his birthday as July 22. The book occurs in the fall), and he also skipped a grade in school. Cherry and Marcia remark that he doesn't look old enough to be going to high school and Pony says he's not.
If the book (and movie) is set in 1966, Ponyboy would have a birthday of July 22, 1952.
There was no kindergarten then, so he would have started in grade 1. He states in the book he got put up a grade in grade school, so let's say he skipped grade 4 just for sake of the argument. So he turns 6 in July and begins school in September:
1 (1958/1959) - 6 (turns 7 the summer after Grade 1 ends)
2 (1959/1960) - 7
3 (1960/1961) - 8
4
5 (1961/1962) - 9
6 (1962/1963) - 10
7 (1963/1964) - 11
8 (1964/1965) - 12
9 (1965/1966) - 13
10 (1966/1967) - 14 (turns 14 in July, book starts in September, turns 15 the summer of 1967; his classmates are already 15 and will turn 16 during 1967)
11 (1967/1968) - 15
12 (1968/1969) - 16 (turns 17 in the summer of 1969)
That puts him in grade 10, makes him the right age and works with the movie. Will Rogers High School (where S.E. Hinton went, and likely where the book was set; it's also where the movie was filmed) went from grades 10-12 at that time, so that makes this the correct scenario for his age.
This also makes Johnny and him in the same grade. Johnny should have been in grade 11 (he was 16 and would turn 17 during his grade 11 year, had he lived), but he was held back a grade. Two-Bit was in grade 11 (it states in the book that he's a junior, still) and Steve is in grade 12 (he is 17 and would turn 18 during his grade 12 year, in April). Soda dropped out, but he was almost 17 and would have been in grade 12 as well had he not dropped out. He likely dropped out during his grade 11 year, when his parents died, or maybe even in the summer, who knows?
Cherry's age is never given - Pony says she and Marcia are "sixteen or seventeen", so she could be in grade 11 or grade 12, depending on when she's born.
(With thanks to S.E. Hinton for straightening this all out for me!)

Tags: characters' ages, ponyboy curtis, ponyboy's age, timeline
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