coin age is a broken idea too.
I don't deny it. Sunny King is well aware of it's problems:
"One of the disadvantages of using total consumed coin age to determine main chain is that it lowers the cost of attack on the entire block chain of history." ... "Another concern is that the cost of double-spending attack may have been lowered as well, as attacker may just need to accumulate certain amount of coin age and force reorganization of the block chain. To make commerce practical under such a system, we decided to introduce an additional form of checkpoints that are broadcasted centrally, at much shorter intervals such as a few times daily, to serve to freeze block chain and finalize transactions." ... "Although the broadcasted checkpointing mechanism is a form of centralization, we consider it acceptable before a distributed solution is available."
Blackcoin doesn't have a solution but a different approach (changing stake modifier) which might improve security:
The proposed changes are intended to improve security in BlackCoins PoS protocol and were made with optimization in mind. With the new protocol possible attack vectors are reduced to a minimum and the incentive to support the network by having a full node running continuously is clearly increased.
The following is dishonest:
The NeuCoin whitepaper addresses all the cryptocommunity's 'nothing at stake' objections to proof-of-stake and demonstrates mathematically how NeuCoin's design - with improved security over previous proof-of-stake implementations - foils all attack vectors.