I have never understood dirt only motorcycles. Useless unless you have many acres of your own property, or have BLM or Forest land next to your property. With a dirt only bike you need to truck or trailer to land to play on, or ride illegally. At 15, you can probably get your motorcycle license/endorsement, and legally ride a dual-sport bike from home to dirt.
I had a 1974 Yamaha 175cc Enduro (what they now call a dual-sport) that I rode for 13 years, in La La Land and out in the Mojave. My uncle went with me, with him on his Suzuki RV125. But, yeah, you are 15 years of age and of course you need a 250cc -- only adults are good enough to ride small bikes fast.