I've been doing front end at my job lately (roughly three years). It is going OK. I did some other stuff too in this period using other front end framework, and some Java-desktop.
Recently my boss asked me whether I would do also back end for some project. It sounded like choice, not ultimatum.
I have small dilemma. I don't see much reason why I would like to learn back end. I prefer to become stronger in front end though (especially in react not the other framework they also use, which is less popular). If I switch to back end also, I see that as it would eat my time which I could have spent on front end and increase my skills in that direction.
I am also not very young, 33. I told him I would think about it. What can I do in such situation?