The age of consent to engage in any form of sexual activity is 16 years old. The Sexual Offences Act 2003 creates offences in relation to rape and sexual activity with children under 13 and children under 16.
Where two children have a relationship with one another and engage in consensual sexual activity, an offence could potentially be made out. However, generally the police will not become involved in cases where the children are of a similar age, unless they become aware of evidence of exploitation, physical abuse etc which exists in the relationship. Action to deal with this situation is usually to provide education and support to the children concerned (for example a school nurse or their GP) may be able to help.
Please see the Crown Prosecution Service website and the other links in the Related Information for further information.
Should you have any concern that your child is being sexually abused by anyone (regardless of that person's age), you can contact your local police force by calling 101. They have a specially trained unit that will help you.