So as far as I am concerned a old person with out teeth can also smile and it is no way connected to teeth. Mr. Mohan if you feel I am wrong you can correct me.
'Idhuvum Kadandhu Pogum "
Even this challenging situation would ease
Keep smiling...one day life will get tired of upsetting you.
But the phrase 'Smile while you have teeth' can be compared with 'Make hay while the Sun shines". It is to mean use the opportunity as and when available otherwise you may not get the chance again.