Low hot water pressure in whole house?

14 year old electric hot water heater is on second floor. Hot water pressure is normal for first 15 seconds at washing machine (also second floor), both sinks and tub faucet (all downstairs) and then drops to half pressure. Flushed the tank, checked for leaks under the house and made sure cold water feed line is opened all the way, it didn't help. Thanks for your time!