Each of mine came to Le Mans once at around the age of 12-15, with a "pre-18, I'll pay - after that you pay your share" rule - it worked and most have been back for repeat visits at varying intervals depending on avaialble funds and other commitments.
It seemed to be about the optimum age for them to understand it, enjoy it, and for me to feel comfortable enough to let them off the leash on their own for a time once orientated, at both the main and other supporting events, particularly as with mobiles, it's easy enough for them to call if they hit a snag somewhere - mind you, the first time revealed that getting their pay-as-you-go phones enabled for Europe before leaving would have been useful.....
MG Mark