The Smart Investors

A person should begin to plan for retirement as early as possible. The earlier, say, in your 20s or 30s, the better because your savings and investments would have compounded by then, though you can start later and do well with consistent savings and smart investments.

I think it should be early enough as of when you begin earning an income.

I would say any time is better than none but the earlier the better since it's all about compounding your money over time.