Girls need to be physically capable of having sex to prevent pain or damage.
You can, but it won’t feel good. A girls vagina will be growing at least some before the first signs of puberty – but it won’t be fully grown for a while. Sex too early will mean that she feels a lot of pain and may also tear internally. Pre-pubescent girls also don’t get wet for sex – so they need to be lubricated.
And, without the hormones associated with puberty – the girl won’t find touching enjoyable.
Its best to wait until a girl has her period before trying sex – it’s a good sign she is physically able to accept penetration.