stewaris If you have reached retirment age and want to go, go. If the company took a sudden downturn, I doubt your manager would think twice about making you redundant if he thought it necessary for the companies survival.
The biggest mistake people make is thinking they are indespensible to their firm. If the work is too much, your manager can always get someone with your level of knowledge in on contract and either keep them on until your subordinate is experienced enough to do the job, or give the job to another person with the required experience now.
Too many people have made sacrifices for their employer, thinking they are essential and their employer needs them, only to find the moment things change they are out on the road with their P45s.