Using oversized lines for any
purpose on a boat is a bad practice. Oversized lines don't fit the hardware such as cleats, they weigh too much, they don't run freely through blocks, and they don't have the appropriate amount of stretch.
Good charts are available that recommend appropriate sizes for dock lines. There's a good chart in Chapman's Guide to Piloting: Seamanship & Small Boat Handling. The West Adviser also has a good chart. The West Advisor: Dock Lines
Don Casey wrote an article for Boat US that includes line size recommendations. Docklines by Don Casey - BoatTECH - BoatUS
I replace my docklines about every 3-4 years, or sooner if one shows signs of chafe. Old lines might look ok, but they become unreliable in a severe storm.