I hit 51 next month, and that&#x27;s ... well.<p>These decadal anniversaries seem to get more onerous each time. 20? No problem, it just means you&#x27;re not a teenager any more! 30? Okay, you&#x27;re officially a grownup, but you can still party! 40 was pause for concern, but it isn&#x27;t what it used to be, the threshold of middle age: that&#x27;s 50. Oops ...<p>Well, as Terry Pratchett observed, inside every old man there&#x27;s an eight year old wondering what just happened. I don&#x27;t feel <i>emotionally</i> middle-aged. But my knees creak in the morning, I&#x27;ve survived medical conditions that would have killed me if I&#x27;d lived a century earlier, I can remember events -- as an adult -- that happened 30 years ago, and I don&#x27;t have the physical stamina, mental agility, or energy of a 30 year old any more. (Mind you, what they say about experience trumping youth and enthusiasm? It&#x27;s not exactly true, but the experience factor makes up for a lot of the gradual cognitive decline that comes with age.)<p>Like Jacques, I&#x27;m less inclined to put up with fools -- but I&#x27;m also less inclined to get worked up over things. And I&#x27;m probably unemployable these days. (15 consecutive years of self-employment will do that to anyone.)<p>But the biggest thing I&#x27;ve noticed? What look like big opportunities, vanishing in the rear-view mirror of age. When you&#x27;re 30 you can think in terms of going back to university or forming a new start-up. You&#x27;ve got time to plan a new career or nurture a business ahead of you. But by the time you&#x27;ve hit 50, the future is looking cloudy. You don&#x27;t want to train for a new career to see you through the next 30 years because who the hell wants to be working to pay off student loans when they&#x27;re 80? So you get locked into whatever track you set yourself running down twenty years earlier. Changing is hard because you have less of a future ahead of you to change <i>for</i>.<p>If you&#x27;re under 35, my advice would be to think about that. Do you really want to be locked into doing more or less what you&#x27;re doing now when you&#x27;re in your sixties? If not, now is the time to change. The later you leave it, the harder it will be.