Is it okay for a 14 year old to do abdominal workouts?

Doing hanging knee raises will not affect your height, it actually helps you grow even more. I would stay away doing sit ups on the floor for now since this can strain your low back if you do it continuously or you are not doing it properly.

Avoid exercised that stretches your rib cage backwards such as dips, bench press etc. This can also stunt your growth quicker.

That's the silliest thing I've ever heard. Okay, not the silliest, but pretty silly. Your height is based on your DNA and your bones. Your ab muscles -- or any muscles will not change what is coded in your DNA or when your growth plates fuse.