What I would say is it depends entirely on your use case which you have not stated here. So to answer your question: yes, yes it is entirely possible to use an old laptop as a NAS. But it depends on what you plan on doing with it. If it's simply to store data, then fine, it should be demonstrably fine for that. However if it's something more complicated like housing a media library that may need transcoding, and streaming perhaps not. If you plan on streaming to say Plex or something, you haven't stated how you want to connect to it either. Will it be plugged in via Ethernet or do you plan on accessing it via WiFi?
When asking such questions, I'd suggest giving as much info so a more informed answer can be formulated.