NO, i dont think it's a good idea due to the following reasons:-
1) Due to the education system prevailing in the country(which is inclined towards producing white collar jobs), the age of getting well settled with a goood job is beyond 21(now a days).
2) we have overpopulation in India, so if the marriage age is reduced, then it can speed up the population growth rate.
3) 18 is a age when we have just crosses adoloscence, entering into the real world, and at this age we dont have much exposure to this outside practical world.
4) if a person is running a business then marriage at the age of 18 might make some sense for him, but for a working(job) person he might get into a job much after the age of 18, and even if he gets a job at 18, then the job would not e permanent.