From this you seem to have drawn the surprising conclusion that there is no benefit to greater in investment in early childhood education, a conclusion which the paper you're citing specifically warns against:
> This finding does not imply that there should be less investment in early childhood programs. There are many early interventions that have large positive rates of return, and there are powerful equity reasons for investment in children.
> The data shows that prevention can be cost effective, but in addition, later treatment and amelioration using evidenced based programs can also succeed.
i don't know if that is true.
i believe there are some things that really make a long term difference, however these things are more likely coming from the parents themselves and less from the schools they go to.
Doesn’t matter which is true. Early educational interventions fade out universally and we have nothing that effects adult intelligence or conscientiousness which explain pretty much all variation in educational attainment, partly through their effect on SES.
> “Heckman curve” update: The data don’t seem to support the claim that human capital investments are most effective when targeted at younger ages.
https://statmodeling.stat.columbia.edu/2019/04/06/heckman-cu...
> The Heckman Curve describes the rate of return to public investments in human capital for the disadvantaged as rapidly diminishing with age. Investments early in the life course are characterised as providing significantly higher rates of return compared to investments targeted at young people and adults. This paper uses the Washington State Institute for Public Policy dataset of program benefit cost ratios to assess if there is a Heckman Curve relationship between program rates of return and recipient age. The data does not support the claim that social policy programs targeted early in the life course have the largest returns, or that the benefits of adult programs are less than the cost of intervention.