RS I don't know how long GC can live, but yours are surely undergrown.
I put mine in June 2013 and they were 1 foot long. By spring of last year, they were over 3 feet long and 25 pounds heavy.
Do you have little vegetation they can feed on?
Be careful because if you have steep banks, when they can't find food around the pond, they start digging the banks with their snouts in search of roots and this causes a slow and dangerous collapse of the banks.