How old should you have to be to...

[DanielX]

Buy birth control? 18 w/o parental consent, 12 or so with
Have sex? Legally?13 with same age group, 18 with any, sliding scale between. Practically? When the kid is ready, which usually at some point between 15 and 30.
Get an abortion without parental involvement?  I think abortion should only be performed when the mother's life is at risk, so it should be handled the same as any other such medical procedure - for under-18 it is preferred, but if there is a provable risk, then any age.
Drink alcohol? Any age, in small amounts.  At a restaurant? 18.
Buy alcohol? 18
Smoke cigarettes? depends on person. I'd say 15-16.
Buy cigarettes? 18
Watch a pornographic film? Soft-core, 12-13. Hard-core, 18.
Star in a pornographic film? 18, maybe 16 if we're talking really soft-core
Drive? 15-16, depending on driving conditions.
Be able to drop out of school? 15 or 16, but "able" is not "should".
Rent your own apartment? 18 overall, younger for orphans and very independent people
Own your own business? 7 or 8 for lemonade-stand things, 13 for a more serious venture.
Go to a strip club? 13-18, depending on how hard-core it is.
Fight a bull? 18, although it isn't a good idea at any age.
Choose to go to a different place of worship than your parents? No legal age. Practical age? around 14-17, depending on the person.

You forgot "Be able to rent a car (at a reasonable price)" which I'd say 18.