When i was 9 i fell onto some glass and a piece of glass went deep into my knee like past the joint
and i still got the scar, is it likely thats the reason of my knee pain?

Maybe you should ask a doctor?

Keep in mind if there are any triggers to the pain, if it's chronic or accute, what helps and doesn't, etc.

You're going to keep the scar too unless you get it removed by surgery..

Is it possible? Yes. Is it likely? I don't think so.

If the glass went deep enough to hit a nerve, it will be damaged. But you would have endured immense pain at the time and it would have bothered you ever since.
There's probably a different reason. What it is, I can't tell you. You should ask a doctor, as Rumblyguts suggested.

Speculation on medical matters is not for these forums, you need to take professional advice - whilst everyone has the best of intentions there is always the chance of incorrect information.