My experience has been that if you've got the chops, you've got no worries at all. Also, fashion is fickle, but right now you'll have people beating down your door to hire you if you're skilled with ML and algorithms.

I am in my 40s.

Thanks for your very encouraging words. You really brighten my day. I've always had an interest in theoretics/foundations and believe a strong mathematical backbone leads to better understanding of these fields. I simply couldn't get that working a normal job, so I know I made the right choice to pursue a PhD. Many, many people recommended against it (especially my friends who are pulling big salaries in industry), but for me it was always about the intellectual pursuit.

If you don't mind sharing, what school are you going to, and what is the area (the more details the better is good)of your research?

In my experience, age is measured from when you finished your education and not from when you were born.

education is a lifelong pursuit.

I'm obviously referring to the last degree that the person received and not to learning in general.