@hyeyeon72 Welcome to Mayo Connect, I am glad you found us, and hope our collective experiences can be of some help to you.
Let's start with "...What’s likely for me to develop cobalt poisoning after 24 years?.." Medicine is a constant learning adventure, not just for us, but for the medical community as well. It has not been very many years since it was found that aging, wearing implants can begin causing symptoms as well as new ones. After 20 years, it would not have crossed my mind that they could be the problem, especially if I thought mine were plastic.
Next, you ask "...How come i wasn’t notified? .." Here we can only guess. I believe the first letters to Metal-on-metal (MOM) hip recipients were issued around 2010. By that time, you were 8 years on, and probably not seeing the surgeon for yearly hip checks? My surgeon handed me the letter during an office visit when I was complaining (again) about issues with my four year old bilateral implants. I truly don't know if his practice would have mailed it to me, and do not know if such notifications were mandatory.

Finally, "...How did your doctor help you?.." He did the metal testing, and arranged for revision surgery. At that time (2011) the manufacturer was paying for this, so he worked with them to cover it. Other than that, he was FAR less than helpful - dismissive actually, of all the other medical issues the metalossis caused. It was my primary who got me to the right docs to manage those. He was so ugly about my side issues that I fired him after the first surgery and found an new ortho, who I still see to this day.

IF it turns out you now have metalossis, you should fine a surgeon who specializes in revisions, and who will listen to and work with you to see if there is coverage for these surgeries. I'm not sure how it would work with implants over 20 years old.
Good luck in your journey, you may need to be the squeaky wheel to get this handled.