The problem in our society is the magical age of 16, 18 and 21.
How would you prefer it done? With a law that can be flexibly applied however way a politically crusading DA wants it to be applied? Would you prefer to be forced to, instead of verifying age, verify "mental capacity", or some other inexact metric?
We have the solid limits for good reasons. It's a defence against tyranny handed down from the legal authorities. The ideal is that everyone knows exactly whether or not their behaviour is legal, so armed with a little information, they have nothing to fear from said authorities. Anything that detracts from this is, IMHO, stupid.
I'd also like to point out that having a (reasonably) developed brain, like an advanced 13 year old, does not imply the knowledge or wisdom to make certain long-term, potentially life-changing decisions. We hope that people have experienced enough of the world at 16, 18, or 21 (in conjunction with the development of mental maturity) to make these decisions. It's not perfect, but it's more perfect than the alternative.