2007 Nissan Altima has a little shake in idle, car has 60000 miles. What could it be?

Plugs can last up to 1000000 miles but it does not mean they would not fail before then. Engine condition and usage can vary the lifespan of the spark plugs.

If the idling speed is too low, or engine mounts are bad, the engine would tend to shakes at idle, more noticeable when the AC is used and gears are engaged.