When I got new carpet laid, I was told to avoid getting it cleaned for as long as possible. Is this correct?

No, dirt is abrasive!

Dirt works similar to sandpaper, every time you walk on your carpets you grind dirt into the fibers, which then cuts the fibers and causes the carpet to wear out sooner than it should.
Dirty carpet simply will not last as long as clean carpet, & while a regular vacuum program helps, vacuuming alone is not enough to remove the dirt deep down in the fibers.
The longer you wait before cleaning your carpet the more damage you do to it and the faster it will wear out.
Always be careful of any carpet sales person that tells you otherwise, as they would have no training or have no certificates in carpet cleaning and maintenance.
The sooner your carpets are worn out the sooner you are spending big $$$ on having them replaced.