I'm not going to let my baby brother use my nintendo switch that I bought with my own money
That seems harsh, why do you not want him to play with it?
He's too young to play those games
Sharing with your brother is a good thing but I can see why you would not want him to play games with him that are not appropriate. Maybe download some games that are for young children and play with him. He will appreciate it and your relationship will be stronger.
But I only have one Nintendo switch.
You will not be using it every time and for those times then your brother can borrow it. You can set limits on the game he plays and it is just necessary for him to also not play those adult-oriented games that you were saying. This way you have your compromise and your brother will thank you!