If I'm not mistaken, there are a few more parameters than just the fee/kB that is included to determine whether a transaction gets included in a block or not.
Coin age is one of those. I think it is possible to send the coins mined and not moved in the early days without a fee and still get high priority when sending them.
As far as I know coin age is no longer relevant. It is just fee divided by transaction size.
Of course it is ultimately up to the miner which transactions are selected.