At the age of 40 am I too old to begin a career in recreational diving as an instructor ..?
I remember asking myself that very question years ago. I was 41 then. I wasn't too old then and you're not too old now. I know a lot of instructors who got their certification in their 30's and 40's. If you have a passion for diving and you're in pretty good physical shape, then go for it. You'll be glad you did.