It's not a truism given that the original comment is implying that the wealth distribution discussed in the article title is a result of people having more wealth toward the end of their life. Theoretically the 10% of oldest Americans could be the wealthiest 10% as well, although this is certainly not actually the case.

Not quite a truism. Given that wealth varies over each lifespan, you should expect somewhat more than 10% of Americans reach the top 10% at some age.

People fall in and out of the categories all the time due to job loss or raises or selling a bunch of stock to make the down payment on a house.

Some people are stuck in the lower quintiles, and some people "stuck" in the higher ones. Other people float around or jump due to life events.

Selling stock to buy a house will lower their stock holdings, not their net worth.

most American have a fair chance at reaching the top 10% by age at some point in the future. not sure of the numbers but I would guess at least 50% of the population is expected to reach am age where 90% of the other people are younger

Shock statistic, 50% of students have below average scores in math!