Practice for English exam.

Topic
Today, more and more young people are starting their own companies. Do you think this is a ("a" is necessary here) good idea?

I think this is a ("a" is necessary here) good idea.
The first reason is that it is a ("a" is necessary here) good experience for young people. They can get lots of ("many" is mostly about something you can count, here we can use "lots of") knowledge ("knowledge" is singular only, there is no plural) about economy earlier (more younger?) than people who start businesses/a business ("start business" is a bit strange in this context, "a business" or businesses seems more appropriate)   after ("since" imply a duration after a past event, wich is not the case here) they became adults.
The second reason is that young people can work freely (in their own business). In Japanese companies, the (as we refer to something/someone precisely, "the" is necessary) boss is usually an ( "an" is necessary) elderly person. Young workers can work smoothly with (by using? -> "with" is good, by using is also goo although the meaning is a bit different) new technology, but elderly bosses (use plural "bosses" or "the eldery bosses") often can't understand it and it takes much time to explain it.
Therefore, I think it is a ("a" is necessary) good idea for young people to start their own companies.

Can someone correct my sentences (many phrases so plural -> sentences) ?
I couldn't express well what l want to say :(. --> I wouldn't say that, everything was quite clear and anyone will understand it :) . There are just a few small grammar mistakes like "A good something", "AN eldery person", "THE company's boss".