My youngest is 10, was a thumb sucker. We bought several devices on the market. None of those worked. She could remove them. And she likes the taste of hot sauce. We also did the taped up gloves, even tried socks! We even considered ordering a dental device. But our pediatrician offered that other than dental damage, thumb sucking wasn't a big deal. Usually, in older kids, its because of some other issue and their need to comfort themselves.
I decided that we really needed to combate the root issue. In our case, we had some major life changing issues happen at the time she probably should have stopped, and was close to stopping. Unfortunately, she ended up regressing....
So, we took a positive approach. I stopped negative words on my part. I reminded her that other people didn't understand that she sucked her thumb because she was uncomfortable. We talked about the things in our lives that changed and how we could comfort each other as a family. I decided from that point on, never ever to tell her to stop sucking her thumb. Instead, I made a point to try to have a conversation with her so that she would have to take the thumb out. I also gave her things to hold...like at a store, please hold this, hold that....and so on.
We also talked about the "thumbsucking" where I would ask if she wanted to stop, which she did...as she was a bit embarassed. We talked about what she could do instead of sucking her thumb, especially around other kids. Ever hear of going to your happy place? That helped her. I told her not to worry, one day she would stop. As for her teeth, I told her that she ought to rotate thumb sucking spots so her teeth won't spoon out.
About six months later she stopped...at about 8 years old. And no dental problems.
So basically these steps helped us:
I stopped saying anything remotely negative about the thumbsucking process.
I reminded her how much I loved her.
We talked about the current changes in our lives.
Found other things for her to do instead of sucking her thumb.
Let her know she can talk to me about her thumbsucking.
Reminded myself, that it was doubtful that she would go to college with her thumb in her mouth.
In the big scheme of things, thumbsucking is not a big deal (rather have a thumbsucker than a bed-wetter.)
Good luck to you.