"Old" means that some Bitcoins are at one address for a long time.
For example, from the aspect of AML risk score, "older" Bitcoins always have a very low risk score and are easier to accept in most services.

Also, "old" bitcoins are a good opportunity to clean money (to the government and tax authorities), because by buying such coins, you justify $110k at a price of $10k (or less) as it was a few years ago.