I think the age of consent should be puberty.
Since a chronological age is needed, I'd say 13, since >90% of people are pubertal by then.
Maybe even 14, if that'll make everyone feel better. Everyone who doesn't have a medical problem has at least started puberty, by that age.
Then- and this is important- I think that it should be a crime for anyone to have sex with anyone under the age of 21, if there's more than a five-year difference in their ages.
Please note that I am not saying, "I think we should declare open season for predators as soon as a child reaches puberty".
What I'm saying is, I think that it should no longer be a crime for a young person to have consensual sex with a partner near their own age, after they've reached puberty.
Now, it may be that parents don't want them to do that. Parents may threaten, punish, whatever. It is up to them to instill morals and enforce rules upon their own children.
But after the age of 13 or 14, I no longer think it should be the state or the government's business to become involved. Some parents just don't care if their sixteen-year-old is having sex, as long as he/she is using protection. Some parents respect their teenagers' rights to form intimate relationships with peers, and to make their own decisions when it comes to sexual matters.
The state has no right to intervene and usurp the authority of these parents.