Nathan is older than Haley. They zei several times that Haley was a jaar ahead of every one else in school & she graduated at 17, while everyone else was 18. Then in season 5 she talks about being 21 & everyone else at that time is 22.
well we know that karen was preg lik in june b4 dan left so luke is feb. so nate is like april. nd so if there is an epi that we kno haley was still 17 when they grad. nd then in 5x04 haley is still 21, which means haley was prob born @ the end of the yr. because there is no way haley skipped a grade, because haley nd luke have been best vrienden since lik they were 8 so tht is 2nd of 3rd grade. and since we know tht in the endish of s5 both nate & haley were 22, haley is prob born @ the end of the yr. i wld say maybe oct - dec.
from wiki: According to Wikipedia:
Lucas was born on March 1988. Nathan was born on June 1988. Peyton was born on July 12, 1988. Brooke was born on March 4, 1989. Haley was born in 1989.
i know brooke is known from 4x02 her birth certificate and peyton from her gravestone from 4x10
the rest not sure how they figured it out. it doesnt make tht much sense to me to have brooke and haley be a jaar younger....