The best time to start planning for retirement is always "now". However, if you're in your 20s, you're probably either in or fresh out of university and faced with a lot of instability. As finance blog Wise Bread suggests, it may not be in your best interest to sacrifice the short term for the long term.
That's not to say that you should put off setting up a retirement plan, but building a safety net for emergencies should also be high on your priority list. Also, consider whether it may be better to pay off high-interest debt sooner. Credit cards and loans that aren't repaid in a timely manner can rack up thousands in interest, which is money better spent elsewhere. Like, for example, your retirement plan.
Retirement Planning If You're Under 30 [Wise Bread]