Without more information, one thing comes to mind and that is a deteriorated or broken dip tube. When this happens, cold water mixes right in at the top as soon as you turn on a faucet.
If your heater dates from the mid 90's, there was a known problem, but it could happen with any unit. Look at the label and check the date of manufacture. Often the month and year is the first 4 digits of the serial.