Can I be a 400lb 23 yo that still plays in the mcdonald's play place?
That seems strange. Why are you playing in the McDonald's play place as an overweight adult?
I don't know it must be because they are not taking me seriously
You should not be playing alongside children in the McDonald's play place. You are overweight and could break the equipment plus it is inappropriate for you to play there. People could be suspicious and call the police.
I don't mind the kids and they don't mind me and I'm having a great time on those slides.
It might be lots of fun, but you should stop doing that. You will damage the structure and you will get banned from even being in the location.