How new does a vehicle have to be for it to have an aux cord jack?

You can retrofit any FM radio with a plugin unit for under $50.
http://www.amazon.com/iSimple-IS31-Antenna-Modulator-Aftermarket/dp/B002U5XPBE/ref=pd_sim_sbs_422_1?ie=UTF8&refRID=17789FX95116ZY5AH26J

Most 2015 cars still do not have bluetooth or Aux built-in. Very few built prior to 2010 have them. You may get lucky and find a few starting in 2006-2007, but you will be severely disappointed if you expect it in any vehicle. They have been a very popular addition and cause for factory stereos to be upgraded, especially by the younger generation, just learning to drive within the past few years.

It varies from model to model.

You can change the stereo for about $200 anyways and get one with an AUX in.

The operative word is new so go down to the dealership and look.

As common stock equipment? I would guess mid-2000s.

You can add a connector onto some factory radios (pull it, crack it open, solder it in, only some radios though), or just replace the factory radio with an aftermarket one with an aux input. My 77 pickup got one with the new radio I put in.

The one I soldered an input onto was in a high-end late-90s car, just ran it through the dash to the inside of the glove box.

My 1962 Ford Falcon has Aux capability because I installed a stereo that has this feature. This can be done with any vehicle in existence. If you want one already installed, it depends on how the car was equipped coming from the dealer. Some cars back in the 90's had aux inputs for their high-end sound systems, possibly even earlier.

you can take the stereo out and buy a aux adapter or remove the stereo and buy a $50 new one (I prefer new)

It varies with brand and the trim level. The fancier models got them around 2006 and not until about 2008 were they more or less universal. And there are still exceptions.