So I was reading the above post (instead of doing some work - this site is great for procrastinating). Needless to say I've never heard of any of this stuff before - age grading, etc.
Anyway, a few years back I did a Race For Life 5k with some friends. My friend and I trained for several months by doing about half the distance - the only time I ran the whole 5k was on the day itself, and it took me 40 minutes. At the time I was amazed that I'd done it, and I still am. Knowing that I did once run that far has helped to keep me going while doing C25K.
I was 35 at the time. Plugging the figures in to runningforfitness.org/calc/... gives an age grade of 37.4. So with a bit of trial and error, I have worked out that now, at 43, I might expect to do 5k in 41 mins 45 seconds. Which is interesting.