When should I start saving for retirement?

The honest answer is as early as you possibly can, even in your twenties with small amounts. Because of compound growth, money invested early has decades to multiply, so a little now can outweigh a lot later. At a minimum, contribute enough to your 401(k) to capture any employer match, since that's an instant return. If you're starting later, don't be discouraged, just save as aggressively as you can and take advantage of catch-up contributions after fifty.