I don't think Walter is implying anything about how common or uncommon this is. His core insight seems fairly objective and plausible to me: "...your chances for happiness are increased if you wind up doing something that is a reflection of what you loved most when you were somewhere between nine and eleven years old". I.e. if you do end up being lucky and wise to do something as a profession closely related to what you *loved* doing when you were ~11 , because you end up spending time doing what you love (and equally importantly not spend that time doing something that sucks up energy) you increase your chances of being happier.

I think you completely missed the point of his anecdote. It’s not a scientific study, he is merely saying that at age 9-11, you’re old enough to have a decent understanding of what you’re interested in, but not old enough to start worrying about social and financial pressures and expectations.

And so the thing you were interested in at that age is probably similar to what you’ll be interested in now, if you remove social and financial expectations.