Right now the software does not allow people to decide which bitcoin one wants to spend. I believe that there are already a number of reasons identified on this forum why that feature would be useful. Bitcoins have a date of creation, and are therefore not all identical.
Why? This would work against Bitcoin by violating one of the fundamental properties of money, that each unit is worth the same as any other (fungibility).
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.