I know my water heater is at least 5 years old, maybe a little older. The last big freeze we had I had to fix the kitchen sink drain (the joint managed to pop loose). We had great water pressure until that day. Now the hot water pressure is ok in the kitchen and bathroom but is only… acceptable, in the shower. Ive tried to purge the water heater by shutting off the inlet, opening the nearest faucet, attaching a hose and opening the inlet but it looks like clean, clear water comes out. Ive purged it several times with no luck. I have 1/2" copper lines coming in and going out even though I've heard 3/4" is better but up until this issue, i have had no pressure problems. Im thinking the water heater is going out or there is a deeper issue then what I can figure out.

Any help?

It is best to go with whatever size pipe is actually on your water heater.  If your water heaters outlet is only 1/2" pipe, it wont do any good to go to a 3/4" pipe.  Are you on a well or do  you have city water?

Im on city water. We want to change to a tankless water heater but don't have the funds right yet to get one and Household six doesn't want another 30+gal water heater due to the low level of energy efficiency.

Remove the shower head and clean out the gunk, if that doesn't work you will need to take the valves (hot and cold) in the tub or shower apart and clean them, if you have the tpyical needle type valve you may have to remove the hose before the valve and clean it. If you really want to make a mess and fix it, shut water off, remove the hose before the valve and blow air though the faucet, shower head  and hot water shutoff.(back flush)

OKC water is extremely hard, the minerals will kill water heaters every 3-5 years where we live.  Our plumber guy would open the faucet at the bottom of the tank and it's big chunks of white mineral buildup, same on the heating elements.  We got tired of replacing the heaters every few years, even though we drained them on schedule as recommended, and installed a water softener system last year.  It's made a world of difference! No more crunchy water, no more pipe and valve damage and much *smoother* water for showers, dish washing, drinking...

The only cure in OKC is a radical flush and purge schedule for your water heater and cleaning faucets and such often or investing in a water softening system.  We got ours at Lowes iirc and did the labor in house, easy peasy.  If you plan on staying where you are and you own, not rent, it might be something you should at least look into, if not for now, then later.

Replace the thermocouple