The earlier you start the cheaper it is to secure a decent pension. Money you save earlier in your working life has more chance to grow than money saved in the years coming up to retirement.
Of course not everyone can afford to save right at the start of their working life with student debts and housing costs to pay. You should also have some savings you can get at before you take on a pension that will tie up your money.
But you should understand that it will cost more if you start later, and you may have other expenses later such as a mortgage and childcare costs as you get older.
In particular, if you have a chance to join a workplace pension scheme to which your employer will contribute, then you are strongly advised to sign up as soon as you can – day one of your job if possible. It's an easy way of getting what is in effect a pay rise, albeit one deferred until you retire.