What do I have to consider with a My Nintendo Account?

So, I entered the wrong date of birth when creating my My Nintendo Account and I wanted to ask for the first time whether the date of birth is important to the My Nintendo Account?

My second question would be, is it possible to change the date of birth? If yes how?

Third question: If I delete my My Nintendo account but I e.g. Bought Octo Expansion (I didn't buy Splatoon 2 online, but as a card!), Will Octo Expansion go away? What about the other games? Can I somehow save them and transfer them to the other account?

First of all, the date of birth is important for verifying your business ability. Ie. Depending on your age, you may not be able to buy certain games online or buy anything at all.

It is not possible to change the date independently in the account / profile.

It is not possible to transfer addons or digitally acquired games to other accounts. Furthermore, the progress in all games is also deleted when the account is deleted.

I think it's best if you write to Nintendo support and ask them to change the date. That should not be a problem.