Interesting. I've just hit 60 years old! (f***, how did that happen?) and many of my age group have already retired, so it is something I do think about.
I've worked freelance from a spare bedroom for over 30 years and made good money ...well, up until 2009 when the crash happened.
What happened to me and may happen to others is that I'm now financially secure. I find interest in other things and my work no longer defines me. It means I no longer stress about being up with all the new technologies, study others' work, look to advance my career and also not worry if no work comes in. I'm very fortunate to be in this position, but I also see it as a privilege of having age (there has to be some benefit, surely!)
I don't need to draw a line under my career, I can just carry on until it completely fades out. The only problem I have is renewing my software every year...I need to be sure there will be enough work coming in to justify it.