In A Game of Thrones it is mentioned that Brandon Stark, Ned's older brother was 20 when he died. It is also mentioned that Ned is 35 years old at the beginning of the book.
Robert took the throne 15 years ago. As the war was followed by Brandon's death, this would make Ned's age 35-15=20 when Brandon died. So this would mean that the two brothers were of same age.
How can this be? How can Ned and Brandon both be of same age?