When did that age rule start anyways? Wasn't Tara 15 when she won?
The rule that skaters must be 15 by the previous July 1 to compete in ISU championships and Olympics was passed at the 1996 ISU Congress. I.e., it took effect for the 1996-97 season.
At that time, there were exceptions for skaters who had already competed in senior internationals but didn't turn 15 in time ("grandfather clause"). Lipinski benefitted from that exception; she had already competed at 1996 Worlds as a 13-year-old, and she was allowed to compete again in 1997 at 14, which was great for her because she won 1997 Worlds.
For the 1998 season she was just old enough (turned 15 in June 97), but it certainly helped her case to go to the Olympics as reigning world champion rather than a first-time competitor at that level.
To bring the topic back to Sasha Cohen . . .
There was also an exception for too-young skaters who had already won a medal at Junior Worlds. Sarah Hughes benefitted from that in that she won silver at 1999 Jr. Worlds (held at the end of 1998) and therefore was eligible to go to 1999 Worlds at age 13. (Actually she went as an replacement for Naomi Nari Nam who had placed ahead of her at Nationals but was too young even to have competed at Jr. Worlds that year.)
The following season, 1999-2000, was the last year that the Jr. Worlds medal exception to the age rules was still in effect. It was also the first year that Jr. Worlds was held in Feb.-March instead of Nov.-Dec.
Cohen placed 2nd at 2000 US Nationals, her first year competing there as a senior. She was 15 years old, but her birthday was after July, so she wasn't age eligible unless she could medal at Jr. Worlds. She didn't manage to do so, so she couldn't be sent to Worlds that year.

