You offer 3 sizes of the flowboard.   Is there a certain size you should buy for riders of a certain age?

No.  The length is a rider's preference.

Why  would I want to buy a 32"  flowboard? 

The 32" flowboards are  lighter and better for doing tricks.

Which flowboard is better for flatlining?

The 42" flowboard is great for flatling.  It is heavier and gives a longer turn. It is a cruising board.  Going down hills, it gives a rider the feeling of skiing downhill.

Which size flowboard should I buy if I want to do tricks and  air riding?

The 36" flowboard is ideal for both tricks and air riding.  

Are the  size  of the wheels different for the three sizes of flowboards?

The wheels are the same size for  all three sizes of flowboards.