I run into this a lot.  A situation that when a person has an older water heater they turn the control or element up.  Doing this just increase’s the temperature and will not increase the amount of hot water.  You will have increased the likely hood of scalding someone.  When a water heater is older in upwards of 8 years or so I would say.  In most cases it is collecting sediment.  This will cause it to be not as efficient, and choke your amount of hot water some.  I like to say a water heater has a life up to 10-12 years on average.  It is a good idea to replace at this time.