I'm not sure about the statute of limitations in Arizona, but I believe it's two years from the date of the death of your father. You say you were a five year old child when he was killed...how old are you now? My guess is that if you are an adult at this time, and are more than two years past the age of majority (18 years old), you're too late.
I suggest you call a personal injury attorney licensed in your state to find out the answer to your question.
Is there anything you can do now? Yes. You can see a lawyer or talk to a lawyer where the events occurred leading to your father's death. Your information is so thin, nobody will be able to give you any definitive answers. More information is needed: your age, what happened, circumstances surrounding your mother's decision (if shemade a decision) not to sue, what made you decide to look into this now, and a variety of other factors. You should see such a lawyer immediately as time limits are strict; and if they have not passed, they are passing with every moment.
Sorry about the loss of your father. Dads are important.
Under Arizona law, you have two years in most situations for you to bring a wrongful death action. As a child, you have a right under Arizona law to bring such a claim. Also, under Arizona law, your rights are not "lost" while you are a minor. The statute of limitations that runs against your claim of two years does not begin to run until you turn 18 years old.
That means that you have until the day of your 20th birthday to file a lawsuit. However, there is a lot of items that would have to be addressed, even if you are under 20.
You will need to hire a lawyer, in all liklihood, to check and make sure no wrongful death claim was filed. It is possible someone filed the claim without your knowledge, as parents, spouses, and children all would have had this right.
You will have to have the lawyer (or you will have to on your own) investigate the case, the potential strengths and weaknesses of the claim, and whether there is any insurance coverage or money that could support the claim. The investigation portion is likely to be more complicated and time consuming due to the delay.
Thus, there may be a viable claim, but there are a lot of unanswered questions that would have to be addressed.