1976 I let a possible purchaser test ride my '64 BSA Lightning. He didn't just wreck it; He ended up riding it into a (fairly deep) pond. Muddy water got sucked into the intake... it damn near completely ruined that bike's engine.
Anyway, I'm a bit wiser today. I will let my son ride my bikes cause I know he can (I would let my daughter as well, but since she's 5'0" she would probably lose it at a stop, she's out).
What could possibly go wrong?