What are the visa requirements for bringing my grandson to Thailand from the UK, and what age restrictions should I be aware of?

Your biggest problem will be getting the child here as you are not the parent. You'll either need court documents giving you full custody, or notarized documents from both parents allowing the child to be taken out of the country, and into your care. It is not an easy process.

Children under 14 cannot be fined for overstay and under 18 cannot be banned.

So get your grandson here using visa exempt and then let him go on overstay.

If his stay here turns out to be long term, and you want to enroll him in school in a few years, you might need to leave the country and re-enter with him to clear his status. But not all schools would even require this and it's pretty far away to think about. For now just get him here with his passport and don't worry about it.