FlightAware - Flight Tracker / Flight Status / Flight Tracking and Global Flight Status and Airport Information, although I don't believe any airlines actually disclose the age and maintenance on their sites. I am not so persistent in doing so these days. Even legacy carrier such as Delta still fly some old DC9s, although I feel they have been well maintained and the company is in the process of replacing these planes (I think they made a deal with Southwest to acquire all or most of the B717s to replace DC9s and some regional jets).
Then, there is always the issue of aircraft maintenance.....many carriers, Allegiant included, outsource all of their maintenance.....even the big guys outsource some of their maintenance these.
So, you roll the dice when you board a plane these days....new shinny paint doesn't equate to new equipment.....who worked on the aircraft last time and where were the replacement parts manufactured?
These days, I have more concerns about my safety on the drive to the airport then the time I spend aboard the aircraft......that said, I still prefer airlines with newer planes.