Many of them are poorly educated having been failed by the educations system. This locks them out of quality job training, which then locks them out of better paid jobs and that locks them out of the housing market... and so it goes.
However:
"Theirs are the shoulders on which the country will rely to provide for a rapidly ageing population, yet they have the worst economic prospects for several generations."
With the rise of the robots, or more likely automated systems as we are now seeing in warehousing, many menial jobs will disappear. The days of the need for a growing workforce are coming, or might well have come, to an end.
Something will have to give in the not too distant future.