But I also wouldn't charge my kids board either, they're my chickens and I'll take care of them as long as they need me to. Although, charging something you will ultimately give back with interest is a clever idea, one I might entertain.
You make it sound like charging older children board is a negative thing - as though it is done to benefit the parents or shirk responsibility. But I don't perceive it this way. My Dad had me pay a nominal amount of my weekly wages to him for board. It was a fixed percentage of my income (which changed from week to week as I was full time casual with penalties for various shifts) - so I always had money to do as I saw fit. My Dad was a single parent and struggling to pay off massive debts incurred by my Mother so I wouldn't have begrudged him asking me to contribute to the house as a full time working adult. It taught me independence, financial responsibility, and budgeting - much easier with a Dad who would say "Ok, you can't pay me your board this week because you stuffed up. But you have to make it up - lets work out a payment plan" instead of a landlord knocking on the door with an eviction notice because I couldn't pay rent. I believe that adult children who are working should stand on their own two feet. If my DD gets a job whilst in school, I won't expect her to pay board - but I would expect her to purchase some of the "optional extra's" she wants (like magazines, movie tickets, those outrages pants her Dad would have a heart attack if he saw on her and so forth). I feel that paying board (whether they get it back or not) is helping them to learn financial responsibility - an essential life skill.