I am new to this net. I just bought a D 5100 because it had a flip screen, when you are past 70years old, you need all the help you can get when shooting down low. This camera requires Nikon S series lenses in order to use auto focus. I am looking at the Nikon 28-300, 55-300, also the Tamron 18-270. Any suggestions would be most helpful.
The answer to your question depends on what lens you have right now and what you plan to shoot, the mentioned lens are all good, but they all require a tripod (as a 70 year old myself, I know what I'm talking about).
The price of the lens are probably a factor too, not to mention filters, lens shades, etc.
Also I believe this camera has a 1.5 ratio, I mean that a 300mm lens equals a 450mm lens on this camera, please keep this in mind when you buy a lens.