What is the measure of my days?

Yesterday we attended a parent's evening at our son's secondary school (for ages 11-18). We got to meet most of his teachers and were encouraged to find out that he has been doing well in his first year. One thing that I could not help noticing is that all the teachers were younger than me! This was a good reminder that life is hurtling by at a rapid pace. It doesn't seem like too long ago that I regarded teachers as a bunch of oldies. A couple of Scriptures came to mind:

O LORD, make me know my end
and what is the measure of my days;
let me know how fleeting I am!
Behold, you have made my days a few handbreadths,
and my lifetime is as nothing before you.
Surely all mankind stands as a mere breath!
(Psalm 39:4&5)

So teach us to number our days
that we may gain a heart of wisdom.
(Psalm 90:12)