I just ran a quick analysis on the blockchain.
Out of the 192386 blocks currently mined, very many of them
have a BTC 50 block reward that has never been spent.
To be exact, 41493 blocks have an unspent reward.
That is worth approximately BTC 2'074'650.00 (not counting fees).
That's 2 Million+ pristine coins hoarded, or about 20% of the BTC market cap.
These cannot be mined in pools, right? Only solo miners can hoard the whole block reward. The "unknown" (presumably the sum of solo operations) typically represents 15%-30% of the hashing power,
http://blockchain.info/pools?timespan=4daysFrom your data sorted by age it appears that in the past year or so only a few percent of blocks ended up unspent.
As we go back in time this goes up to 40-50%. Seems like the change takes place with the advent of GPU and pooled mining.
Provisional yet shocking conclusion: miners hoard a lot, but your method only reveals hoarding by solo miners.