Verbal Agreement between roommates?

My roommates & I had a verbal agreement for me to not help pay the utility bills They wait almost 2 years without a word, till.. I shared a place with 3 guys. I later landed a job in another state. Before leaving I told the roommates that I wouldn’t be around to be a burden on the utility bills and wouldn’t use anything as far as water, heat, electric, etc. I never had anything plugged in while I as going to be gone. They wished me well & said they wouldn’t expect payment of the utility bills since I wouldn’t use an ounce of electricity, water, etc. (verbal agreement) I returned several times over the past year & a half & talked with the roommates about various things. We had friendly conversations each time about every topic imaginable. Not once did they mention anything about me helping pay any parts of the utilities. I renewed my lease under the pretense everything was ok. now they want back payments. What to do? They could have contacted me through email address they had of mine, through the landlord, they had my phone number, and access to my social networking pages. They allowed the bills to go a year and half with not a word mentioned. We sign separate leases for our responsibility of our own bedroom. We also have an electric and gas bill which is from outside utility companies which we are to handle on our own. The apartment complex has no dealings with the utility companies, yet think I owe 1/4 of the bills. I stated that I was never there as a burden to create any bills. I simply used my room only as a place of storage. These guys didn’t approach me about 1 bill. Before I moved out I paid 1/4 because I was there once or twice a month yet I was still there. They didn’t even seek to mitigate their damages until 1 1/2 years later. When I moved out I never spent another night there or used any appliances. What should be done and what is a fair resolution?