I think that, financially, it's probably never too early to start planning for retirement - I'm sure that any financial adviser worth their salt would say that the day that you get your first wage packet is the day you should start saving and investing for your retirement. (I read an article the other day that talked about teaching teenagers the importance of retirement savings!)
As far as the best time to begin planning the non-financial side of retirement is concerned, I'd say that:
1. It's never too early to start making a bucket list - a list of things that you want to be, do, have and experience before you 'kick the bucket'.
2. I think it's also never too early to start to work on improving your health - so that you have the energy and strength to tackle your bucket list and ensure you have the fittest and healthiest retirement you could possibly have.
3. I think it's never too early to pay attention to relationships - particularly if you have a tendency to give priority to your work, rather than your family and friends. If you think that, once you've retired, you'll have plenty of time to do all that 'relationship stuff', you can, sadly, sometimes find that you left it too late and your relationships are damaged beyond repair.
4. I'd say that two or three years out from retirement, you need to start paying particular attention - I think that this is a good time to enrol on a pre-retirement course because it can be a consciousness-raising experience which can get you thinking about all aspects of retirement - some you may never have considered before.
5. Two to three years out is also a good time to start to put out feelers about your options at work: Do you want to finish work completely or would you like to stay on doing part-time work? Can you negotiate a phased retirement, whereby you begin to gradually reduce the number of hours you work each week - a client of mine is a teacher and she's just in the process of negotiating her working hours for the next academic year - she's 62 and the school that she works for is quite anxious for her to stay on, but she thinks she can see a way to reduce her working days from five to three, whilst still giving them what they need from her.
6. If you want to continue working in some capacity but don't want to stay on with your present employer, two to three years out is a good time to start thinking about what you want from your post-retirement work and to start to look around for the types of jobs that would fulfil those wants and needs.
There's a saying in retirement coaching circles that 'most people spend more time planning their annual holiday (or vacation) than they do planning their retirement', so, if you pay attention to the points I've mentioned here, you'll be ahead of the majority of your peers.