How long do I have to claim compensation for medical negligence from when I was a child?

[Laura]

I had multiple fillings done by my dentist when I was a young child, they were done poorly and have caused me trouble and pain ever since, I am going for another appointment soon to see what can be done about them.
I am turning 18 soon and this must have happened 5 or 6 years ago, how long do I have to claim for dental negligence?

Thank you