Bitcoin Forum

Does the chance for a transaction fee decline if you use old coins? I don't get it -why?

New coins have slightly higher fee, because it's more likely new coins are used for sending in small amounts aka spamming blockchain and network.

Someone might come to elaborate more technically and correctly, but I think I'm not too far wrong.

Transactions two days and older (or something like that) are no different as far as fees go.   

If you want to do massive amounts of spamming of tiny transactions, you can but you are going to end up spending your change transactions to do so.  If there are higher fees for these, that means you really need to pay a relative fortune to carry out the pay spam attack.

So what is the formula for determining if something will require fees?

You'll find the priority formula here: https://en.bitcoin.it/wiki/Transaction_fee

Keep in mind these are not really mandatory. Any miner may ignore such rules and create a block with a different transaction policy.
These rules were hardcoded in the Satoshi client as a method to prevent transaction spam, in the absence of something better. I hope one they these rules are replaced by an actual anti-spam code, but that's not easy and I guess there are more urgent development priorities.
As most nodes respect such hardcoded fee policy, you'll have a hard time broadcasting a transaction that does not respect it - most peers won't relay it. If you really want to ignore this policy, you should try to send your transactions directly to the miner that ignores it too.

The idea is that someone trying to overload the network will find his coins getting newer and newer while someone trying to get a legitimate transaction through will find his coins getting older and older. This will quickly allow the legitimate user to beat out the attacker and make it very hard for an attacker to clog out legitimate transactions for very long. This tradeoff reduces the average transaction fee a legitimate transaction needs to pay.