Would insurance be lower for a 2000 Ford Mustang GT rather than a newer one (2005-up)?

I'm currently 18 years old resident in Dallas, in still in high school, and planning to paying fully on a 2000 mustang gt. How much do you guys think my insurance would be a month? Liability insurance

Generally, the older the car, the cheaper the insurance. Especially since you want liability insurance, it would be relatively inexpensive.

Generally yes. Older cars tend to be cheaper to insure because they aren't worth much. It eventually gets to the point where it's cheaper to have liability only coverage because the premiums for comprehensive coverage would cost more than the car is worth. Where you live, your driving record, other people in your area's driving records, where you park the car, your grades, your/your parents credit score, and several other factors will determine how much you're going to end up paying.

It will be high.
Liability says you are happy to walk away if anything happens to your car.
There are lots of events where it is not your fault but your insurance ends up paying.
Odds are you will crash it on the first year.

How about calling a car insurance agency and finding out if they will even insure you?

If you have to ask, you probably can't afford it. Pick up a phone. Make some calls. Local insurance agents are happy to give price quotes.

Hi so miracles take a time the answer to this is just impossible.

Probably older car will be cheaper but not always. Much depends or how they calculate costs and risks. Bottom line will be what is cheapest option for them will set your rates.
apart from factors such as age. Area you live in. Local crime rates. Mileage covered, is car stored securely etc the cost of replacing the car used to mean that newer cars will cost more. However some newer cars are much safer and offer better passenger protection etc and as in many case the real high costs will be payouts for injuries and death and not replacing metal .paint and rubber etc. A newer car can at times be in fact cheaper. Only be actually getting a quote will be really see what out charges are.