Is it true that cancer in younger people grows much quicker whereas its slower in older people and if yes why (i couldn't find a answer online at all?

Different cancers have different growing rates. And different people have different cancers. I can think of no reason that younger folks would have faster growing cancers other than the type of cancer the have. For example certain leukemias untreated kill kids in days or weeks. Can also happen in adults but much rarer. So it will depend on the type of cancer and the factors driving it.

No, not true. Every cancer is unique. Some grow quickly, some less quickly, in both old and young people.