No, of course you're not too old!
I'm not sure you need a helmsman's course though to steer a narrowboat through the British canals. You need a radio if you're going on the Thames and obviously if you're taking your narrowboat into coastal waters, but if you're doing that you'd be better off with a different sort of boat.
We hired a narrowboat for a week and the only instruction considered necessary by the company was about an hour being shown how to turn the boat round and work the locks.
For several years after that, we owned a share in a syndicate boat and had some lovely times in most of the English canals.
Narrowboats are very narrow though and can feel a little claustrophobic - I would try it out if I were you, before committing to anything.
Canal Boat magazine is a good source of info, and Terry Darlington's books Narrow Dog to Carcassonne, Narrow Dog to Indian River, and Narrow Dog to Wigan Pier are a good read.