In reply to keith-ratcliffe:
I am sure that if you ask 100 climbers what is their limiting factor, 99 will answer 'finger strength'
also it seems that 6a/6b is a usual level that people get to without any specific training and then progress slows down for many. So I think age doesn't seem to be affecting your climbing in any unusual way so I wouldn't think about doing anything too specific 'for your age'.
Having said this, obviously be extra careful with injuries as they will obviously knock you out for longer than someone 50 years younger - so I would take things slowly as advised above. Also you will naturally find it a bit harder to put on muscle and if you don't train you will probably have a somewhat faster muscle wastage, so my guess that your best bet is to make sure you climb frequently and mix things up ie do a lot of easy climbs too.
Nutrition is also important, I don't know your build obviously but don't forget that keeping weight in check is as important as getting stronger.
Personally I stopped worrying about finger strength and just make sure I climb a variety of different problems focusing on technique, seems to help me a lot more than strength training did. If you want to get stronger, I would do lots of bouldering if I was in your shoes before trying finger boards and strength training...