Health insurance at 21

I had gotten a secondary health insurance without knowing that my main one didn’t allow that, which is my parents health insurance. So I got kicked off of it. Well I had no idea until now and I didn’t re-apply for the secondary insurance so now I have nothing. Even though I’m 21, could my parents add me BACK onto their insurance? I know I can remain on it until I’m 26 but I just don’t know if I would have to be under 18 to be added, or if they can add me at any time before I’m 26. This may be a silly questions lol but any input would be nice. I don’t know if it depends on the insurer or what