I think so, My mother asked for my boyfriends number and its the exact same ages, im 16 hes 19, its just a precaution incase there an emergency. I think you should
My daughter is 16 and her boyfroiend is 19. He's lovely. They go out a fair bit and she is generally pretty good at answering her moby. I would just like the option of being able to ring him if it came to it. Is it a reasonable thing to ask?
I think it kind of depends on how long they've been together, it depends on whether or not it might make things awkward in my opinion. If you can be sure he'd think nothing of it then sure, why not? but is y your daughter going to think it's a littler bit stalkerish? it's up to you tbh.