Is one ever too old for a quality phone?

I hope not, because I just upgraded my 72 year old mother from a Nexus 5 to a OnePlus 3. :) Well, it's on the way.

I may not modify anything on the phone. However, I do have a question: Does OnePlus update their phones to the latest versions of Android often, regularly, or will she be lucky to see updates at all?

Thanks!