A Treatable Issue, Or Old Age?

[Bearclaw]

I have a recently-sickly betta. I got Apollo from a pet store a little over 3 years ago. He's always been really healthy, but obviously he's getting up there in age. I'm not sure if what I'm seeing is an illness, or just old age. For the last 3 weeks, he came up to the top of his tank at feeding time kinda "side-saddle", instead of straight up - like he was having trouble swimming. Over the last week or so, he has really deteriorated. his fins stay drooped down, he has lost color, particularly on his underside, and he lays on the tank plants facing the surface, or on the bottom, even laying on his side at times. He can dart up to the surface if he tries, but it's like he can't stay there. Like he is no longer buoyant. I don't know if he's eating, he doesn't show interest in it when I try to feed him. I've done the treatments listed below and several water changes. I don't know if I can help him, or if this is the end... :(
Any advice is greatly appreciated.

Housing
What size is your tank? 2.5 gal.
What temperature is your tank? 72 F
Does your tank have a filter? Yes
Does your tank have an air stone or other type of aeration? No
Is your tank heated? No
What tank mates does your betta fish live with? None

Food
What type of food do you feed your betta fish? "Micro-granules" only
How often do you feed your betta fish? Once a day - evening

Maintenance
How often do you perform a water change? Generally 2-3 X a week
What percentage of the water do you change when you perform a water change? 30-40%
What type of additives do you add to the water when you perform a water change? Stress Coat

Water Parameters:
Have you tested your water? If so, what are the following parameters?
I must confess, I don't check it myself, I take it to the pet store. Yesterday, they told me it was "perfect". I don't know the values below.
Ammonia:
Nitrite:
Nitrate:
pH:
Hardness:
Alkalinity:

Symptoms and Treatment
How has your betta fish's appearance changed? He has lost some color from his normal all-red, particularly his underside and forward to his head.

How has your betta fish's behavior changed? He began a few weeks ago by coming to the surface sort of on his side, rather than oriented vertically. Now he lays on one of the plant leaves constantly facing upward, or on the bottom curled up. He can dart to the surface momentarily, but he can't seem to stay there - as if he has lost his buoyancy.

When did you start noticing the symptoms? About 2 weeks ago.
Have you started treating your fish? If so, how? I did a 5-day treatment of something called 'Tetra LifeGuard'. Also 'Tetra Algae Control', since there was some green algae forming on the tank.

Does your fish have any history of being ill? He has always been very healthy. The past year or so, he has had trouble hitting his food granule on the surface (he misses slightly), but I figured that was his eyesight going bad. Other than that, no problems.
How old is your fish (approximately)? I have had him a little over 3 years since I got him from the pet shop. So who knows, he could be 3 1/2 or even 4 years old, i suppose. So I may just be witnessing his ultimate natural end, but I want to help him if I can. :)

[ZackyBear]

Try bumping up the temperature to 75+ He could be a little cold~

Does he seem bloated in any way? I'm thinking SBD, but the age part makes me a little unsure...

[MoonShadow]

Try bumping the temperature up to around 80, 72 is a little cold for bettas, especially one getting up there in age. When they're in colder water they can stress and their immune system can take a hit. Some hobbiests claim that epsom salt can help give a slight boost to the immune system so it wouldn't hurt to add a little to your tank.

[Bearclaw]

Thank you for the responses! I'm going to grab a small heater today.
Would epsom salts work the same as 'aquarium salt'?