Ages of consent

[Sooga]

Like many people, I don't see the point in age-based requirements to do certain things. For example, a guy aged 20 years, 364 days can't win a big jackpot in Vegas, but if he just waits a few more hours, he's perfectly fine. Having sex with a girl aged 17 years, 11 months could conceivably get me some jail time or at the very least a fine (here in CA anyway), but if I wait a few weeks, I'm in the clear.

The fact that in this country different states have differing ages of consent is a testament to how subjective it is.

On the other hand, there are pretty clear instances of when things shouldn't be allowed. For example, I can't see any scenario where it should be ok for a 10 year old to walk into a store and buy some hard liquor, or to have sex with a 12 year old.

My question is, in a totally ideal world with perfect information (in other words, you know what everyone is capable of doing/thinking), how would you determine when someone should be allowed to do certain things? Making an age-based requirement is almost completely arbitrary, but I can't really think of any better alternatives. But surely there have to be some, right?