Does is appear to be leaking from where the oil sump and engine block come together? Is so, it's probably a leaking gasket that can be changed pretty easy. The sputtering and smoking could be a dirty carburetor or even that could you possibly be overfilling it with oil to compensate for the leak? Excess oil has to go somewhere, so it goes into the carburetor. Smoke will tell you a bit more of the story: black is a carburetor metering problem, white is oil being burned.

The real answer to your question is based on whether there is internal wear inside the engine that may be the reason for this. If you have access to a compression gage or a leakdown tester a cylinder reading would provide helpful diagnostic information. If you don't have that, you can make an educated judgment by removing the spark plug wire and engine cover, holding back the brake lever or tieing it down, then spinning the flywheel by hand counterclockwise (backwards) until it builds up the most compression. It should spring forward sharply if your piston, cylinder, and valves are in good shape. If they are, a few minor parts should get you back in shape.

However, most mowers are designed to last about seven years on AVERAGE. Some will last less, some considerably longer, based on maintenance, cutting conditions, and plain ol' luck of the draw. So you are on AVERAGE going into what would be expected as the end of the life cycle on your machine.

Hope this helps!