Katie,
A few of us have zoomed past the 80 mark (years not mph) and are still effortlessly shifting our 4 speed stick models. My guess is that I will be taken off the road for some other, as yet unknown, reason before I get to the point of not being able to deal with the stick shift.
Practically speaking switching out the standard transmission for an automatic is a no-go and after becoming attached to your 230SL, I think it will be difficult for you to make a switch. So, just keep on shifting.... or buy yourself a second Pagoda with an automatic box!