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. 

It CAN work, but I’d talk to your DM like they said just to be sure.

The youngest in our group actually was playing a very young ELF of all things which was very interesting to me, but some members of the group recommended he bump up the age to have it make “more sense.” Whatever. I personally thought there was a neat story behind it. He seemed like sort of a runaway druid once involved in a cult (our characters do not know about the cult part yet). I’d like to think my character has partially taken him under his wing (no, I mean he has wings. Lol) while another in the group gives him a harder time.

The player is arguably more new than I am, but sometimes that’s where the best ideas come from is somewhere unexpected. Anyway, enough harping on that; I think I’ve made my point.