Water Heater - temperature erratic

I have a duplex that is 10-years old and has the original hot water heater. Lately, I've been noticing that the temperature of the water is very erratic. It starts off hot but in a few minutes goes to luke-warm and I have to keep turning it hotter and hotter until I'm out of hot water (about 10 min). It never goes completely cold but i can consistently seem to keep a temperature while showering. Some days I notice this more than others, so it's not even a consistent problem. Do you have any advice as to why this may be happening or what I can do?

Gas or electric?

At 10-years, you are on borrowed time.

Gas heater - what am I looking at as far as things that could be wrong?

Sounds like the dip tube which delivers cold water to the lower portion of the tank is going bad. This allows the cold water to mix higher in the tank reducing the volume of hot water for your use. As noted above, 10 years puts you on borrowed time.