Is Age Related to the Change in Map Prices?

The following graph plots median prices for items listed in the Antique Map Price Record, over its 20 year history, by the age of the item.  Clearly the median prices for early items (pre 18^th-century) is higher than for 19^th century items.  But it appears that the increase in prices in recent years has been steadier and sharper for the 19^th century items.