...is there a way for me to know how old my coins are ?
The previous information about priority failed to mention that priority also takes into account the size of the transaction. Priority is actually calculated as:
priority = sum(input_value_in_base_units * input_age)/transaction_size_in_bytes
For the purposes of calculating priority, input_age is determined by the number of confirmations received since the bitcoins were received in a transaction. input_value_in_base_unite is determined as the number of "satoshi".
So, If you received 2.3 BTC in a transaction that now has 72 confirmations, then those bitcoins would contribute the following to the sum in the numerator of the calculation:
230,000,000 satoshi * 72 confirmations = 16,560,000,000
If that was the only input in the transaction and the transaction was a total of 276 bytes in size, then the transaction priority would be:
16,560,000,000 / 276 = 60,000,000
If a transaction has a priority less than 57,600,000, then most miners will refuse to add it to a block unless it also includes a transaction fee of at least 0.0001 BTC per kilobyte.
and will older coins be more desired over time ? like rare or collectible coins...?
No way to predict what others might choose to value in the future, but I generally find it unlikely since the act of sending them the bitcoin would immediately reduce its age back to 0.
I suppose you could give them the private key associated with the address that the old coins are associated with, but that seems VERY insecure. Since you also know the private key, you could steal the bitcoins as soon as you parted ways.
