It is not a question of why. This feature can easily be implemented. So it is just a matter of time before it is there. After that the market will decide the relative value of bitcoins with different ages.
Note that old physical coins also have a collectors value. Bitcoin differs from old physical coins in:
1. that it does not have a weight in metal. Note that the coinage is only a certificate of authenticity. Note that all atoms of a metal are quantum mechanically completely identical.
2. that it is not subject to legal tender laws
3. they do not wear out.
I meant why would older Bitcoins be more valuable as a currency? It makes no sense at all. If two denominations of 50 BTC are not worth the same as each other, it's a huge strike against Bitcoin as a currency.