The children were selected to represent the range of socio-economic backgrounds in Britain at that time, with the explicit assumption that each child's social class predetermines their future. Every seven years, the director, Michael Apted, films material from those of the fourteen who choose to participate. The aim of the series is stated at the beginning of 7 Up as: "Why do we bring these children together? Because we want to get a glimpse of England in the year 2000. The shop steward and the executive of the year 2000 are now seven years old."
56 Up premiered on British TV on 14 May 2012; its release in the USA came on 4 January 2013.
You can read more about the series at the Wikipedia link, below I will post you tube videos in sequence.
Because the first two documentaries are split up into parts, I will link you to them with external links since I dont want to embed that many pieces.
7 UP (3 parts)
21 Up (7 parts)
There is also 42 Up, 49 Up and 56 Up which was recently released, however I wasnt able to find proper embeds for those documentaries. I suggest searching for them online or purchasing a digital copy.