For a start it is called "Health Equalization" because health inequalities are very marked in the perinatal (not neonatal) period and in infancy. But between around 14 and 18 (depending on what paper you are reading,sometimes it goes higher) years of age it seems that health inequality is indeed lower than after around age 30. It does depend on how you measure health, however. Social class differences in average height, for example, are just as great in "youth" as earlier or later in the life course (or they were last time I looked). Class differences in, for example, louse infestation are also large. It is in mortality that we really see the smaller health gap.
Well, not many people, thank goodness, die during adolescence (this is changing now, but it was true in the 1980s and 1990s). At this time, cancers were one of the more common (not common really, just more common than heart disease which was the biggest killer in the adult population) causes of death in adolescents and cancers do not generally have a big social gradient. Apart from lung cancer, and this is unknown in young people. Leukaemia, for example, did not at this time have much of one as far as I remember.
The importance of the idea, however, was in the light some authors thought it shone on how health inequality in adulthood emerges. If, they argued, inequality in health according to the social class of one's parents was low in adolescence, but inequality according to one's own occupational class then emerged strongly by age 35 or so, maybe this tells us something about it. Perhaps it is that there are unhealthy adolescents scattered through the population at random, regardless of their parents social class. During youth, the deaths of these unhealthy adolescents will not, therefore, show a health gradient. What causes inequality in mortality later on in the life course is that the unhealthy ones (who were going to die anyway) could only get the lousiest, worst paid and most hazardous jobs? So although it looks like low pay and job hazards cause early mortality, actually it is poor health which causes both early mortality and having a lousy job. I know this sounds ridiculous, but that is what the argument was and why it got so much air-time.
I remember talking at a meeting to some people from a support group for people with chronic kidney disease. One the the conference speakers had put this idea forward. The people with kidney disease were very amused. They asked if anyone had the slightest idea what it was like, and how silly the idea was that people like them would be selected into things like mining, building work and ship building. They were looking forward to telling all their friends about the ludicrous academics who thought building workers have high mortality because they already suffered as children from diseases like kidney failure.
One thing you have to remember, especially in today's economic environment, is that the statistics on mortality that were being used did not include people with permanent disability who had no stable occupations at all. You could only be included in the statistics on health inequality if you had an occupation which defined your social class. So people who had chronic illness from childhood that prevented them from working were excluded altogether. And this was also in the days before people with chronic illnesses were being forced into various types of low paid work. So in fact, people in hard jobs were selected at the beginning of their working lives for good health. This has been called the "healthy worker effect". But that is another story.
As it was, at the time, there were quite a few people, including policy-makers, who for quite some time believed that health inequality was due to this process whereby sick people were recruited into tough jobs. It was called "direct selection".
The most important papers were written by Patrick West & colleagues:
West, P. Health inequalities in the early years: Is there equalisation in youth? Soc Sci Med 1997 (44(6)) pp 833-858
West, P. Macintyre, S. Annandale, E. Hunt, K. Social-class and health in youth - findings from the west of scotland 20-07 study. Soc Sci Med 1990 (30(6)) pp 665-673
and David Blane & colleagues:
Blane, D. Bartley, M. Smith, G D. Filakti, H. Bethune, A. Harding, S.
Social patterning of medical mortality in youth and early adulthood. Soc Sci Med 1994 (39(3)) pp 361-366