Having the same concerns, though I'm only a 30 years old. From my experience, 2 hours with a 8.7 lbs (3.9 kg) Peavey Foundation on a strap hurts my left shoulder.
My fretless 34" Yamaha RBX250 weights about 6.7 lbs (3 kg) & I definitely feel the difference, though I've switched pickup to Lace...