Because 65 is the assumed retirement age, it's the one people are least likely to actually run the numbers on — they treat it as the point where things simply work out. Sometimes they do. Often, though, a plan built on "I'll just retire at 65" assumes a pot the contributions were never going to build, and nobody notices until it's late to fix.
This calculator is the reality check: enter your age, savings and contributions, set a return and an inflation rate, and see whether the money lasts through a retirement that could run twenty-five years or more. Everything is shown in today's money, so a big future balance doesn't fool you into thinking you're further ahead than you are. The pre-filled example is a 40-year-old heading for 65 — put your own figures in. The projection is free; the deeper survival testing against bad markets is a paid extra. Just try it, because 65 is worth checking rather than assuming.