I've published more information about these two ladies on my Rector's Wife Revamp blog, but here are two of my favourite inspirations for fitness, no matter what your age.
Renee Clarke (born 1926) - you'll find an article about her here on the Rosemary Conley website. She's based in Surrey, near where I used to live, and seems still to be taking part in events with her local running club, the Epsom Oddballs.
Jenny Wood Allen (born 1911) - here shown on the BBC website (scroll to the end of the article), but you'll find references to her in many places on the internet. From a quick Google, it appears that Jenny last ran in about 2001; since then, she's walked the London Marathon in 2002, received an MBE in the 2006 New Year Honours List, and took part in the 2006 Great Scottish Walk.
So for those of us in our 40s and 50s, there's very little excuse!