Well, this is interesting. UK law differentiates depending on where the parties live:
The age of consent for sex
England and Wales
The age of consent to any form of sexual activity is 16 for both men and women. The age of consent is the same regardless of the gender or sexual orientation of a person and whether the sexual activity is between people of the same or different gender.
It is an offence for anyone to have any sexual activity with a person under the age of 16. However, Home Office guidance is clear that there is no intention to prosecute teenagers under the age of 16 where both mutually agree and where they are of a similar age.
It is an offence for a person aged 18 or over to have any sexual activity with a person under the age of 18 if the older person holds a position of trust (for example a teacher or social worker) as such sexual activity is an abuse of the position of trust.
The Sexual Offences Act 2003 provides specific legal protection for children aged 12 and under who cannot legally give their consent to any form of sexual activity. There is a maximum sentence of life imprisonment for rape, assault by penetration, and causing or inciting a child to engage in sexual activity.
Scotland
The age of consent to any form of sexual activity is 16 for both men and women, so that any sexual activity between an adult and someone under 16 is a criminal offence. The age of consent is the same regardless of gender or sexual orientation.
There are possible defences if the sexual activity does not involve penetrative or oral sex. These are if the older person believed the young person to be aged 16 or over and they have not previously been charged with a similar offence, or the age difference is less than two years.
Sexual intercourse (vaginal, anal) and oral sex between young people aged 13–15 are also offences, even if both partners consent. A possible defence could be that one of the partners believed the other to be aged 16 or over.
A range of specific offences protect children under 13, who cannot legally give their consent to any form of sexual activity. The maximum penalty could be life imprisonment for rape, sexual assault, sexual assault by penetration, or causing a young child to participate in sexual activity. There is no defence that the accused believed that the child was older.
Northern Ireland
The age of consent to any form of sexual activity is 16 for both men and women. The age of consent is the same regardless of gender or sexual orientation.
The Sexual Offences (Northern Ireland) Order 2008 introduced a series of laws to protect children under 16 from abuse. However, the law is not intended to prosecute mutually agreed teenage sexual activity between two young people of a similar age, unless it involves abuse or exploitation.
Specific laws protect children under 13, who cannot legally give their consent to any form of sexual activity. There is a maximum sentence of life imprisonment for rape and assault by penetration. There is no defence of mistaken belief about the age of the child, as there is in cases involving 13–15 year olds.
Article 79 of The Sexual Offences (Northern Ireland) Order 2008 amended “relevant offence” for section 5(1) of the Criminal Law Act (Northern Ireland) 1967 to exclude the duty to report information about the commission of an offence under Article 20. This therefore means that there is no statutory duty under criminal law to report to the police cases of sexual activity involving children under the age of 16 under articles 16 to 19 of the Order, where the other party is aged under 18.
This exclusion does not apply to information about offences against children under 13, as set out in Articles 12 to 15 of the Order.