How old should you have to be to...

[Verily]

Buy birth control? Any age
Have sex? Any age
Get an abortion without parental involvement? 16
Drink alcohol? 18
Buy alcohol? 18
Smoke cigarettes? 18
Buy cigarettes? 18
Watch a pornographic film? Any age
Star in a pornographic film? 18
Drive? 16 (cars only)
Be able to drop out of school? 18 and must have passed sophomore year of high school
Rent your own apartment? Any age
Own your own business? Any age
Go to a strip club? 18
Fight a bull? () Any age, I guess
Choose to go to a different place of worship than your parents? Any age

[Verily]

How credible would it really be for a 16 year old to run a business? And how successful do you think any such business would actually be? Seems like to much youthful dreaming and little reality around here, sometimes.

It's possible. In high school, I knew a girl who ran two businesses of her own and made quite a bit of money doing it. Of course, she was very exceptional, but that's not the point. There's no reason to lawfully restrict at which age you can run a business.