Consent is an interesting subject.
Girls have their period around 12 years of age on average, but the process of puberty spans a few years, and there is a lag before ovulation becomes regular and mature as a process.
I think however this is quite irrelevant to the idea of consent. Consent is something that honours the right to autonomy of a conscious being, and valid consent requires competence on the part of the individual to understand and retain the relevant information allowing a truly informed decision to be made. So when talking about consent we are talking about the mind of the person rather than bodily functions. Whether a girl is physiologically able to get pregnant or not is irrelevant to it, otherwise one might consider a woman who is amenorrheic (doesnt have periods) to be unable to consent to sexual activity, which is absurd. (many conditions can cause you to not have periods, and genetic causes are usually responsible for never having a period from the word go).
As for consent, i think it is entirely an individual thing. Some people can be younger and be able to validly consent, whereas others will be much older. Mental health and learning difficulties is something that poses many difficulties here.
Of course the law states the cut off, and it must draw a line in the sand somewhere, but realistically you might get a 15year old who is much more competent at making such a decision than a 17 or 18 year old.
In medicine, with regards to treatment, you can as a doctor, if you feel that the child that has come to you with a medical problem is able to grasp and understand the relevant information surrounding it and the treatment available, you can consider them 'Gillick competent' and treat them without informing the parents. The term 'Gillick competent' is from a legal case back in the 1980's.
I think it illustrates how, with respects to consent, and with respects to a fixed arbitrary cut off, one must philosophically be aware that there are no absolutes in this world, and that valid consent revolves around an individuals ability to absorb, understand and retain the relevant information regarding a specific situation, to which they then may be in a position to make an informed decision.
Now, as for how consent can be effectively communicated is another problem!
Alex