The house is 13 years old. One of the two baths on the second floor runs out of hot water much sooner than the other. Is it possible that the shower valve is not working properly?
Are there parts in the valve that can be replaced to remedy the problem.
I followed the instructions in message 16041 from Feb. 18, 19101 and made no progress for more hot water.
I assume that you've checked that all fixtures are out of hot water when you run out at the upstairs shower? My guess is that the showerhead upstairs is using more water than the one downstairs. Newer ones, and valves are designed for a max of 2.5 gallons/min. Older ones, or ones without (removed?) restrictors can use considerably more. My unprofessional opinion.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013