When you pass 60 and realise you could be in the last quarter of your miserable life.
When your child becomes 40.
When your grandchildren are getting married.
When you watch your Countrys rugby team running onto the field and wish you were one of them but know you will no longer have the chance.
When your grandchildren approach you for your 'long in the tooth' knowledge.
When the walking is not really a problem but getting out of the chair to go for a walk is.