I'm missing major pieces to academics?

I can't do simple things because I refused to work as a child. ABCs? Forget it. I know all the letters in the alphabet, just not the order. This is the same with the months of the years and simple mathematics. For example, I'd have to do out 36 + 59 completely. It's honestly awful and embarrassing. I'm currently 16 years old and I want to revisit these skills so I can master them and be relatively smart around academics. Is this still possible? Or am I too old to do it? If I can, how can I go about it?