SAN DIEGO -- There is a thin line between experience and ineptitude. And, I'd argue, seasoned politicians cross that line all the time.

I'm grateful to Sen. Dianne Feinstein, D-Calif., for reminding me of this important life lesson. Whether she realized it or not -- and I'll put my chips on "not" -- Feinstein has sparked a discussion about how long is too long to be on the job.

The spark occurred when the senator was recently confronted in her office by an angry mob of adolescents and pre-teens who seem to have gotten a little carried away by the quaint notion of representative democracy.

Will someone please sit these youngsters down, give them snowcones and explain that all that "of the people, by the people, for the people" jazz needs to be taken with a truckload of salt?

The young activists wanted Feinstein to do something radical: earn her paycheck, and soon her pension, by representing them.

The issue at hand was climate change, and the young people -- who were likely devotees of Rep. Alexandria Ocasio-Cortez, D-N.Y., who has said the world is on borrowed time -- demanded that Feinstein support the bill referred to as the "Green New Deal" in order to save the planet.

"Some scientists have said we have 12 years to turn this around," a girl told the senator.

Feinstein bluntly schooled the kid on the facts of political life, explaining why it's unrealistic to think that Congress will do anything about climate change in the next decade or so.

Then the senator told the young activists that she doesn't respond well to folks who tell her that she has no choice but to do things their way.

It's true. Politicians don't like being told what to do -- unless, that is, the folks giving orders are holding checkbooks. Then, they don't seem to mind so much. And the bigger the check, the more accommodating the politician.

Feinstein also told the group that she has "been doing this for 30 years" and that "I know what I'm doing."

But does she really? I've covered Feinstein for most of those 30 years -- including on such issues as immigration, education, gun control, the Iraq war vote, and more.

Trust me: She often has no clue what she's doing. She long ago went from being part of the solution to evidence of the problem.

And you know why that is? It's because job experience is a double-edged sword. The trick is to do something long enough so you get good at doing it, but not so long that what you get good at is not getting anything done.

No matter what kind of job we're talking about, you want to pick up expertise, not bad habits. You want to effect change, not fortify the status quo. You want to find ways to get results, not come up with excuses for why you're coming up short. And you want to stay the course and not get stuck in a rut.

When you've been a teacher, a doctor, or anything else for two or three decades, having a ton of experience can easily go from an asset to a liability.

Just look at my father, a retired law-enforcement officer who was on the job for 37 years. I used to tell people that his tenure was long enough as to make him totally unqualified to do anything else. For instance, at some point, maybe in the 1980s, he crossed the invisible boundary where he could no longer serve on jury duty. When the defense attorney would ask him if he could be impartial, my dad would smile and say: "Not really. If your client is sitting at the defendant's table, he must be guilty." He was soon dismissed.

I suffer from a similar affliction. I've been writing professionally for nearly 30 years. And, after spending all that time covering politicians skilled at spreading manure, it's a wonder I still vote. Recently, a reader urged me not to be so cynical and so distrusting. As he put it: "When you spend all your time in sewers, pretty soon, everything looks like a rat." That explains it. I see rats everywhere.

The popular theory advanced by Malcolm Gladwell, the popular writer and thinker, says that it takes 10,000 hours of practice to master a craft.

What no one tells you -- and what someone should tell Feinstein -- is that it's the 10,001st hour you need to worry about.