Jeez, why do people have to question the metaphysics of straightforward and legitimate good questions with real, non-trivial answers?

This is a question where superior Zen-line pronouncements on life philosophy are not just wrong, but are dangerous in the sense that they encourage people to underestimate the importance of genuinely important things.

There ARE critical ages and decisions you make that have a disproportionate effect on the future. And they are not surprising either.

Ages that are of common importance to most people and widely recognized as such. What people don't get at these ages is just how important these decision points are. Typically you understand their importance 10 years later, alongside realizations that are regretful to greater or lesser extents. "If only..." is a bad thing if you like wallowing in self-pity, but a good thing if it increases self-awareness and appreciation for how life actually works.

1. Choosing a basic "personality" for your teenage years at age 13 or so. Are you going to go geek or jock? Are you going to consciously cultivate social skills or some sort of introverted creative skill? Are you going to master the status game or the impact game? Are you going to be the funny type or the serious type? We make most of these decisions around 13, and stay locked in until about 18, because teen years are fairly brutal in not allowing you to change your mind (unless you change schools). With 5 years of practice these attitudes become nearly impossible to change later. There is some reinvention in college, but not as much as people think.
2. Choosing where to go to college at 18 (among whatever choices you have) is a hugely important decision. If you don't go to college, the 18 point merges with the 22 point (below).
3. Choosing where to go to live and work at 22-23 after you graduate is also a hugely important decision. Richard Florida's work shows that the city you pick is far more important than the specific job you pick for instance, because of the way creative capital and relationships work.
4. Transition to "mid-career" somewhere around age 35, with or without a mid-life crisis. This is a subtle one and doesn't get written about much, but at some point you realize that Act I, designed for fresh young people, is over. If you get a fresh start, it is still going to be Act II and have the history of Act 1 behind it. You cannot directly compete with the Act I types anymore. Most people sleep-walk through this transition, or agonize through it without making any real changes. They foolishly get into denial about their Act I and try a second Act I, competing with younger people and throwing away their experiences simply because it may have been unpleasant. Or they try to continue with no changes. The smart way is to take stock of Act I and consciously think about how Act II is different.

Ages that are different for different people, but in the same rough range:

1. Picking a basic life direction and values (the 1-sigma range is probably 25-28 today, but rapidly drifting up). If you don't pick one, you'll default into one whether you like it or not.
2. Deciding whether or not to have kids (typically 35-40)
3. In America, the age when you buy a first home, if you do buy one (it is a huge enough decision that it can completely pwn your life from then on)

There are many other life-changing events that seem like they can happen anywhere/anytime, but in practice these events have more statistical structure than people like to admit. Take mentors for example. Finding important mentors is nearly always life changing. And sure, it might be a homeless, drunken Zen philosopher you meet in some seedy Tokyo corner at age 45, but it is far more likely to be a good teacher you encounter in high school or undergrad, or a manager at one of your early jobs.

Sure, many things are changing radically as middle-class scripts break down and young people extend their explorations of early adulthood into their 30s. Sure, not all parts of the world run on such timelines. A kid in a wartorn African country has a different schedule.

But biology matters. There are enough elements of the script that are determined by biology rather than context that the concept of "crucial age" is an important one. And there's simply the matter of there always being less time on the clock with every passing year, making younger years more important in leverage terms.

So in summary: 18, 22, 35 are the most important sharply-defined ones that are shared by most people. There are a few more individually variable points in the 20-40 range.

Beyond 40, statistically, your leverage and control over the script systematically diminishes. You can always find some idiot motivational speaker telling you you can turn things around anytime. The 1 in 100 who does will serve as the poster-success for the next round of suckers. 1000 years ago, hucksters used to wander around selling fake immortality elixirs and this is basically the same thing.

So don't be dumb. Realize that people pay attention to certain crucial ages because they ARE in fact important. Realize that these "control points" for your life get less frequent and less leveraged as you age.