J.C.
You can't put an age on when you'll stop providing guidance to a child. Even as adults, maturity levels are always different. Heck, I have a brother who will be 30 in 5 months who's as immature as an 18 year old.
You never stop being as worried and as involved in your kids' lives. I say this after considering my mom's relationships with us kids, and also my husband's parents' relationship with their kids. They still help out my husband's brother's family financially, and he's 37. As far as my mom goes, she's always getting calls from us for advice. It never stops, and thank goodness for that!!!
I think the only time you have to "decide" whether to continue supporting a child, whether it be financially or emotionally, is if they're full grown adults, and it gets to the point where you've become the "enabler" rather than the guidance guru.