Young character

In the adventure we play, my character is an adult fighter. but in the next adventure, i want that my character is cursed and now a 12 year old kid. are there any rules to changing the stats?

Nope. Talk to your DM.

Also, talk to your DM and table about the whole concept. This kind of thing comes up sometimes, playing as a child, and curse or no, not everyone is comfortable with it. It also risks you being the main character, as the rest of the party tries to protect the child.

Of course, that kind of stuff is table dependent and some folks might have no problem with it, but it’s a session 0 discussion.