There is only one answer to this question: it is illegal for two minors, or an adult and a minor to have sexual contact.
Specific to this question, the boyfriend at thirteen is old enough to be tried as an adult. Many before him have been, many after him will be. It may not be right, it may not be fair, it may be an attempt by some to legislate what others claim to be their nautral developmental drives, but the reality is, if a twelve year old and a thirteen year old have sexual contact, and it becomes known, there is a very real probability that the thirteen year old will be charged, tried, convicted, serve some sort of incarceration, and register for the rest of his life as a sex offender.
So, ask yourself: is five minutes of experience worth a lifetime of pain?
In the UK, it is also illegal for 2 minors or an adult and a minor to have sex. In the UK, a minor is anyone aged under 16. There is also a risk of pregnancy and STDs, even though you are young and even though you may not have started your periods yet. Don't be fooled into thinking that just because you haven't started your periods yet you can't get pregnant or get an STD