Thinking out loud..
If a coin has an 'expiration date', then the older it is, the closer it is to expiration, so the LESS it is worth. This sounds very similar to demurrage ? I can see why Impaler invited you
Trying to keep track of how old coins are, whilst keeping the same value, seems quite complicated. I think that the demurrage principal used by freicoin, where the coin is worth :
(It's value) * (-5% per annum), is simpler for people to understand.
Your idea of buying newer coins with older ones seems THE SAME as simply applying demurrage to all coins ?
The newly minted coins will be worth more since the demurrage has not had a chance to affect them as much.
What I do see as different is that if you had an exchange for older to newer coins, the AMOUNT of demurrage would effectively be set by the market, and would not be set in stone.
This could be a useful metric in deciding the state of the economy, and whether money needs to be injected or not. Hmm..
So, what if the coin amount inflated by 2% per annum, or some amount. BUT the amount of demurrage that was applied to each coin was decided by a some kind of on chain exchange. Similar to exchanging old coins for new.
If the demurrage was set to more than 2% you would have monetary deflation, otherwise inflation ?
I'm not sure if the money injection would be done per coin, or just given straight to the miners.