We have an issue in Britain right now with our shipbuilding and manufacturing industry. A severe lack of apprentices. As in we are literally watching our ability to do high skilled (as in well done) manufacturing of things like ships, electrical gear, engines, and other things die as the people who have theses skills exit the workforce without passing on their skills.
If everyone above 40 were barred from working our entire skill pool would probably dissolve overnight and the UK's remaining industries would collapse near instantly. Dumb idea put forward by actual fucking retards who think everything is learned either via university; google, or is unskilled and can be picked up easily. That's not even getting into the issue of everyone over 40 suddenly not being able to afford housing, food and social life; which would annihilate the economy overnight as well.
If you kicked out the over 40's you'd literally cause the most pervasive and disastrous economic depression in the history of the UK. If you did it worldwide you'd literally collapse the world economy in its entirety. Probably to never really recover.