A 2-3 year old betta fish is definitely not an old one. Betta's can live up to 7-10 years with the proper care. Most betta's die at around 2-4 years of age due to improper care and unsuitable environments. I don't recommend anything under 5g for one betta fish with a gentle filter and a heater set to around 80F.
The large white bubble indicates a parasite. If he's in an unfiltered container or even a bowl, that's why he's suffering. Parasites thrive on an abundance of excess food, combined with stagnant water. I recommend getting him into a suitable environment ASAP.