The standard answer to this question is "as early
as possible," and while that's technically correct, it's also not very
useful — because most people asking it aren't twenty-two, and many have already
let decades go by without any real plan in place. Telling someone in their
forties or fifties that they should have started sooner is accurate and
completely unhelpful at the same time.
So here's a more honest answer: the best time to start
planning for retirement is whenever you're actually going to do it. And that
time is almost certainly now, regardless of how old you are.
The reason early planning gets so much emphasis is
compound growth — money invested at thirty has decades to grow in a way that
money invested at fifty doesn't. That part is real and worth understanding. But
the implication that people who didn't start early have somehow missed their
window is wrong, and it causes a lot of people to avoid planning altogether
because it feels too late to matter. It doesn't. The decisions you make in the
ten to fifteen years before retirement often have more impact on how retirement
actually goes than anything that happened in the decades before that.