I found myself saying the above cliche to my husband this evening…. I can’t even remember the context. I shouldn’t have said it. I hate the phrase and all the other quasi compliments and expressions relating to age that are bandied about.
Have you ever noticed how hard it is to find a joyful, friendly birthday card for anyone over 40 years? You end up having to settle for a blank card with a pretty design on the front.
What does “not bad for your age” mean anyway? That you are good for your age?Does it mean that we should lower our expectations of what you can do, say, look like? Is it all we can say of a positive nature under the circumstances?
Sometimes I like to embellish this post with wise words and I can almost always find some on a quotes web site; here’s a couple:

The great secret that all old people share is that you really haven’t changed in seventy or eighty years.  Your body changes, but you don’t change at all.  And that, of course, causes great confusion.  ~Doris Lessing
What most persons consider as virtue, after the age of 40 is simply a loss of energy.  ~Voltaire

Interestingly, the above site has more things to say about ageing than any other topic I’ve looked for recently. Most of the quotes, sadly, are deprecating.
I like what my friend Collyn, in his early eighties, has had as a ready reply for age detractors: that he is “not bad for any age”.
The beautiful thing about yoga is that it is for all ages. The baby in utero, her mother, and her grandmother. Kids in school and older kids in grad school. Sportsmen, mechanics, and musicians. Retirees and renunciates.
At every age and stage, yoga is a springy system and will bend to your needs. Not only that, it will help foster your well-being, sweet nature and long life. And perhaps it will give you enough detachment so you will be merely amused by any belittlers.

Burmese Granddad