You are certainly never too old.

Although my dad doesn't race these days, apart from the occasional orienteering event he's probably doing more miles 'training' than me at 76.

Talking of orienteering if you do feel that you are too slow to race over the fells you've got plenty of years ahead of you in that sport- I came across the results from the World Masters Orienteering Champs the other day where they had Over 90's age races with the oldest competitor born in 1914.