Yes what is the reason? I know the age of consent is lower in other countries. What is their reason for that?
OK. Long story as short as I can make it. (Not very short.)
When the 2003 Act was drafted, there was some suggestion that the age of consent should be lowered to 14 or 15. (Most of Europe has set it at one of those; in most of the rest of the world it's 16 - higher in a few places.)
No reason was given for keeping it at 16 - it was just stated it would stay the same. Reading between the lines, however, and considering the aim of age of consent laws generally, I think it is probably to increase the robustness of the law in terms of protecting vulnerable young people.
The Act does make clear that it is there to protect "children" (my quotes - child in the legal sense), not to punish them.
The main danger is seen to be from adult sexual predators, but there is an acceptance that some young people might be at risk from their own peer group. However:
The Crown Prosecution guidelines state "[I]t is not in the public interest to prosecute children who are of the same or similar age and understanding that engage in sexual activity, where the activity is truly consensual for both parties and there are no aggravating features, such as coercion or corruption."
This statement shows clearly that the government accepts that there can be consent below the age of consent, which seems nonsensical until you realise that there is another, lower, age of consent -13. Sexual activity with someone below the age of 13 is an absolute offence with no defence acceptable.
But the acceptance of consensual sexual activity between 13/14/15-year-olds is an example of what is known as a Romeo and Juliet clause. Other countries have similar laws - New Zealand, for example, and The Netherlands (one of the few European countries where the age of consent is 16). Personally, I think it sends the wrong message. "You're breaking the law but we're going to let you get away with it."
A better way, in my opinion, is the close-in-age exemption, where the law doesn't apply in the case of truly consensual sexual activity between young people of similar age. Canada and South Africa use this system, according to Wikipedia.
I think many countries are moving towards 16 being the standard. Spain raised their age of consent from 13 to 16 recently, and Croatia from 14 to 15 - in both cases with a close-in-age exemption.
One thing is clear. The law is not there to stop young people from being sexually active as long as it is really what they want. The law is there to protect them from having sexual activity forced upon them against their wishes.
And let's not forget, under-16s have been able to access contraception without their parents' consent or even knowledge, since the Gillick case of the mid-1970s.