In my long observation of car values and prices, I would say around 20 years old is about the bottoming out point. Look at the prices actually being got for ED9 CRX's now compared to 5 years ago (there are always exceptions) but they will level out for probably another 5. By that time the attrition rate will have made them really rare and prices will start to rise.

When I was into the Valiant Chargers in the 1990's I struggled to get $1825 for mine (a nice example). I would get 10 times that now (even my wife reluctantly agrees I should have kept it.)