This rule has been there since ages, dont know the exact time.
The reason for this is that the olympics are not on the official FIFA-calendar so they originally limited it to young players (whioch are not always key players in their clubs). But with time the pressure on the FIFA increased from IOC site and that rule got a bit spongy...allowing 3 players older then 23 years...and because there is so much money around with a football tournament with the best players there will definately be an olympic tournament with those players either 2012 in London or 2016.
The reason for this is that the olympics are not on the official FIFA-calendar so they originally limited it to young players (whioch are not always key players in their clubs). But with time the pressure on the FIFA increased from IOC site and that rule got a bit spongy...allowing 3 players older then 23 years...and because there is so much money around with a football tournament with the best players there will definately be an olympic tournament with those players either 2012 in London or 2016.