What does a comparison of Bitcoin Days Destroyed say about Bitcoin vs Namecoin vs SolidCoin? The Abe blockchain explorer provides statistics for percentage bitcoin days destroyed, using the generic term "% CoinDD".  Currently Bitcoin is 36.1245%, Namecoin is 41.5547%, and SolidCoin is 47.0952%.

My guess is that the relatively high numbers for Namecoin and SolidCoin could indicate that people are cashing their mined coins in for other currencies or goods more regularly than people have historically done with Bitcoin.  What is known about the Namecoin and SolidCoin transaction cultures?

But of course Bitcoin only became really active in recent months.
It would be interesting to see comparisons of recent activity.  Are regularly updated graphs over time available for these % CoinDD values?
While Bitcoin DD% is lower it is a lower % on a larger base.  Converted to a standard unit say USD Bitcoin transactions are a magnitude higher than other coins.  The lower DD% doesn't tell us anything about the nominal amount of transactions but rather the relative days destroyed between currencies.  

The lower DD% simply means that less of the older coins are moving in the bitcoin economy than other economies.  Other economies are much newer thus they don't have very old coins which due to their extreme age can skew the stat donward.    If the oldest SolidCoin miners simply mine and hold they at most can prevent a 14 days from being destroyed.  For early bitcoin miners holding onto a single coin can prevent hundreds of days from being destroyed.

As for reason why older coins may not have the same velocity as newer coins.  Some of that may be loss.  For a long time bitcoin was worth less than a cent.  Someone with 10,000 coins (a huge sum tody) may not have taken very many precautions against it and lost coins in data losses or system formats.  

Some early miners may have done it simply for academic reasons rather than any investment and when they stopped they deleted (intentionally or accidentally) their wallets.

In the early years of bitcoin there wasn't much value or reason to quickly sell coins.  Now the miner may no longer even be mining and just holding the coins as a speculative investment.  Newer chains usually have an exchange open from day one.  Checking the alternative cryptography forums and exchange chats will indicate a large number of miners aren't mining alt coins for long term appreciation (which would result in low DD%) but instead are quickly selling on exchanges for other currencies including fiat.