wow! lots of attention recently...
If an exchange waits for 6 confirmations, you would only need to split it in 6 (maybe 7, not 400 like I did).
I don't know why I wrote this... I lost focus... actually you need to split the stake as much as you can.
That's the key to the thing: if you split your staking then your staking weight remains the same. But splitting gives you the advantage that on each block you find your staking weight is only marginally affected. If you split in 6 and you hit a block, you are left with 5/6 of your original weight. If you split in hundreds then each block you mint doesn't affect your weight because the age destroyed is minimal. This is why I could do 90 blocks with no extra effort.
I'll try to write a paper about the countermeasures to defend against this kind of things... but it's not directly related to coin age: coin age only makes it cheaper to attack, so coins like blackcoin or nxt can still be attacked with less than 10% of the supply.
And I'll do nxt next (testnet or a clone) but it'll take some time because it's a very different thing I need to get used to.
And a warning: most POW coins are not safe! it's easy to cheaply rent enough hashrate to attack a lot of coins with low to medium hashrate. Only the biggest ones are good.