Signs of aging …

From: Martin Van der Linden
In chapter 1 page 6, you mention the case of the effect of start age
on results in school as and example of fundamentally unidentified

Do you mean that what cannot be assessed experimentally is the very
effect of starting school later because a student who starts school
at 6 and is perfectly identical in all dimensions but start age to
another student starting school at 7 cannot be found?
If true, is there any reason we would like to measure this pure
effect of start age independently of maturation effect? Isn't it
precisely maturation effect we try to measure when thinking about
start age?

yes! any first grader who started at 7 will be older than a first grader
who starts at age 6 on test day.
Since we think there are big age-at-test effects,
the comparison in test scores between these two is misleading

Many school districts would like to boost their test scores and are
tempted to go for an older start age to do it. 
Older start ages will indeed boost scores
(suppose you couldn't enter first grade until after your bar mitzvah ...) 
But that fact doesn't mean older entrants are learning more; they
might well do worse (e.g., by virtue of the dropout age mechanism
detailed in Angrist and Krueger 1991)

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