As always, IANAL. And there probably is CA case law specific to roommates and contracts and civil statutes of limitation; I'll give someone else on LSE a chance to search for relevant cases and post that.
You are solely responsible to the utility company since you are named on the account (which I assume is the reason for your question), unless you are married. But if the roommates were sent bills - either in letters or simply posted in notes the door of the fridge - in the past and they paid them, and you have records of those payments, you can argue that they formalized the contract they had with you concerning the percentage of the utilities they paid while they were living there, and have now broken the contract for the utilities when they were residents. This also goes for rent payments; by paying rent, they formalized the contract they had for room rent and living there.
Broken contracts are civil matters, and there are varying statutes of limitations for contracts; and those limits can depend of the last date of the roommates' residence and the bills you sent, and also the date of any civil filing you may do.
Re: your question: it's not really personal injury, and it's not really rent, either, though I guess in a legal sense, utility bills could be associated with rent. (That is, unless part of your dispute really does concern rent).
Depending on your interpretations (and the court's, too if you end up there), it's more likely either or both of these statutes:
Contracts: Written: 4 yrs. §337; Oral: 2 yrs. Civ. Proc. §339
Collection of Debt on Account: 4 yrs. (book and stated accounts) Civ.
I'm not going to offer legal advice on what you should do or when the statutes of limitation expire in your case, but take a look at http://lawhelpca.org/ if you are low income or want to find someone to talk with. And also look at small claims court in your county; they often have basic legal help.