Is it okay to start majoring "Behavioral Science" since mid 30s?

Go for it!! Im 45 and going for my Master's in teaching. Don't wait! But behavioral science, I would make sure you have more focus. Get a license like a licensed behavior therapist or school psychologist...who also studies behavior. I did consider getting my license for behavior therapy, but there are few jobs where I am, unless you live in a large metro area.

Do what you are passionate about! That is where you will find life's greatest treasure and happiness. I am 27 and just now starting nursing school. I want to become a NP but will be in my 30s by the time i get there. I recognize this but still keep my plan because it is what I am passionate about. Do not make your future life decisions based on how you perceive others may view you. Do what you are going to be happy doing... it will reward you greatly. Best of luck in your studies and future career! :)

You mean "majoring in" behavioral science "in your 30s"? I don't see why not, but
perhaps need to get a firmer grasp on prepositions.