The keypool, as the name suggests, is a pool of keys stored in your wallet.dat file for various functions including:
- change addresses
- new addresses
By default you'll have 100 keys. Each time you use a key for change or receiving, there'll be one less key in the pool. A new key is added to replenish the used key.
The oldest key (from February 2014) represents the age of the oldest key. In other words, you're storing BTC at an address that was created in February 2014. Replenishing the keypool doesn't update the date because that would mean the private key for the oldest (and still useful!) key has been cleared.
There's no inherent issue, per se, with old keys, if they're secured.
(Although encrypting your wallet will replenish all keys since a compromised wallet would reveal a sizeable portion if the now encrypted keypool)