I'm 40yo/5'8"/200lb/30"inseam/strong like bull. I got my first dualsport last year. A DR650SE (367lb curb weight) is about as heavy as I want to deal with solo in sand and mud. A low 225-350, with a mid-range powerband, that weighs 300lb curb or less, would be easier to learn dirt on, for sure. An XT225/250, CRF230L, or KL250 Super Sherpa come to mind. If you're longer-legged people than me, a taller 250 under 300lb could be easy enough too...KLX250S or DR250/350SE. These kinds of dualsports are typically MUCH easier to ride than a heavy feet-forward bike.
Wear full gear that will work offroad. "Riding shoes" don't cut it. Use supportive and protective BOOTS. A sturdy leather 10"+ workboot, or the Fly Racing Maverik ATV boot is about as low/minimal as I would go. It fits well with knee/shin armor. The moto-style Maverik, and the inexpensive Oneal Element, are taller. Basically...Protect your head, hands, chest, back, arms/shoulders, and legs from impacts. You will almost surely fall while learning dirt. You will fall repeatedly. It's part of the fun.