5 Gears usable out of 6
A bike 18 months old is very unlikely to have a friction shifter.  Did you mean its an older bike that you bought 18 months ago?

Raise the bike off the ground, either in a stand or hang it from a tree branch or rafters by rope.  Then slowly pedal with your hand while pulling on the inner gear wire whereever you can get to it (front downtube is normal)

You should see the Rear Derailleur move through its whole arc of motion and the gear change should happen.  This would isolate the problem to your shifter's mechanism.

If the gear change doesn't happen, the problem is at the back. 

Consider posting some photos once you've had a closer look.