- Firstly, I admit I can't find where I heard the age 35 specific claim to reference it. I have listed two sources below and found an even stronger claim by Thomas Sowell (that most people at one year of their life in America earn top 20% income), but it wasn't that statement I was looking for, so I will keep try to find it. But income decile movement surveys in the UK supports this, as do dynamic income surveys conducted in the US, so I'll share those to you:

- Income is highly mobile. When tracking what income groups have, within groups there is great variation. Half the people with top 20% of income drop out of the top 20% over 5 year periods, with a significant proportion of those dropping out (30%) dropping to below average income. In other words 15% of the people earning the top quintile of income will earn below average income in 5 years time.

... Meanwhile, showing upwards mobility, 20% of the people earning the 20% lowest in the UK moved to the top 40% of income by just 5 years later in this study (Figure 19 / corresponding table) - https://www.gov.uk/government/statistics/income-dynamics-201...

- You seem to assume income goes up over time, however in western countries, income tends to peak at 30-50, hence 35 in the stat. It peaks at age 37 and stays flatish until it declines from age 44 in the UK, for example: https://www.statista.com/statistics/824464/mean-disposable-i...

- Historically, older people earned more in the 40s and 50s, but due to COVID or the simple growth in proportion of people economically inactive or in part time roles at those ages, you can see how in the UK in the last 20 years that trend disappeared: https://www.ons.gov.uk/peoplepopulationandcommunity/personal...

- Children and pensioners essentially earn nothing for the purposes of this stat, but as they still spend (or indirectly through parents consume) money, they're in this total. Pensioners are reasonably part of this stat because they are spending in the economy, but are seldom members of the "working population"

The main resource I would suggest on income mobility is "Chasing the American Dream: Understanding What Shapes Our Fortunes" which is based on the Panel Study of Income Dynamics (PSID) [https://onlinelibrary.wiley.com/doi/abs/10.1111/jacc.12716] - it does date to the 70s/80s, but it focuses on: can you achieve top income, can you have income security. Their major finding is that A) Most people through randomness and effort end up earning above-average incomes during the middle stage of their life, and that B) Wealth, but not income, is cumulative.

This is also historically constant, for example Thomas Sowell wrote about how most Americans at the time would be in the top 20% of income for at least one year of their life in "Economic Facts and Fallacies" and "Basic Economics". (obviously the UK quintile study notes decreased lower income exit mobility which is bad for poorer people and means dynamic income mobility is reducing).