To start with, I don't consider 30 to be old!
I am 52 and I started riding about 5 years ago. My first bike was a 1986 Yamaha YZ-125. I love that bike but I would not recommend it for a first time beginner.
When I started riding it never occurred to me that I might race, yet here I am competing in Enduros. I suck at it, but I have a great time regardless. When I started competing the YZ-125 was no longer suitable so I upgraded to a 250 two stroke.
You should keep in mind that dirt bikes come in a range of styles. The "top end" are the MX bikes, which are built for racing/jumping/track use. They are built light and lack luxury items such as kickstands. They are also expensive new but the two strokes get fairly cheap when they still have a lot of life left in them. An example of a MX bike is a Yamaha YZ-xxx
The opposite end of the spectrum are the "off road" models, such as the Yamaha TTR-230 or Honda CRF230. These bikes have detuned motors that will tolerate lower octane gas, are significantly heavier, substantially less suspension travel and are much cheaper (like half the cost of a MX bike).
In between there are "cross over" or "dual sport" bikes. A dual sport is capable of being street licensed although the street tires suck in the dirt and the dirt tires are not legal on the road. Dual sport licensing varies state to state so a bike that is legal in one state may not be in another.
What I would suggest for you is to look for a used "off road" model, such as the Yamaha TTR-230. These are four stroke models and are easy to learn. For your size I wouldn't go any smaller than 230cc. Plan on keeping this bike for a year or two and then trading up to something bigger such as a YZ-450F or a WR-450F.
Alternatively, you could look for a dual sport bike that is licensed for the street. The dual sport would allow you to ride those jeep trails and use the public roads to get from one trail to the next. I don't recommend riding your dual sport with dirt tires in the city or freeway, however.
As far as the brand, all of the major manufacturers build a good bike so it is basically pick your favorite color.
Rod