Q. Which is better in terms of money and mileage for 18 year old boy - Continental GT 650?

Well, you can own a gt650 at 18, i myself being 18 own a gt650, i had an experience of a 40 year old bullet 350, which gave me total confidence of buying gt650, so if you are ready for heavy bike then go for it you wont regret it.

If you are comparing Continental GT650 and Interceptor650 and if you are experienced and if you can handle a bike which is around 200KGs in weight then continental GT will be a better choice......but if you did not have experience of a 200-250cc bike then I won't reccomend you....but if yes then go for gt650

For a 18 yr old ....give him thenderbird 350x or classic 350.......conti gt should be his second bike ...after he gain more experience....