I retired about 55 and am now about 62. Live in Aus, currently in UK after taking 7 months to drive the Canter here. We don't know how long it will be before we return to Aus. Our constraints are bureaucratic (visas etc.). Average around 150 km/day but would like it to be much slower. Since we didn't have too much rest (only 20% of days with no driving) we are enjoying a rest now.
We are not as fit as we once were (though that's all relative and we still walk up steep mountains relatively quickly) and have noticeably less energy. Whereas we used to drive and sight see in the same day now we are more likely to drive one day and sight see the next. We also rely a bit more on the comforts we take with us.
As we've grown older our confidence has diminished a bit. We are much less likely to take some of the risks we did when younger. Not to say we are risk averse, perhaps just more careful. We describe the 4wd and recovery gear as there to get us out of trouble not into trouble. Having said that perhaps we hang on to the thought that what distinguishes us is what we do when we have problems rather than avoiding them. If we ever lose the sense that all problems are solvable it will probably be time to stop.
Our travel experiences are such that we are generally aware of feelings of insecurity in alien (to us) cultures and have a sense when to remove ourselves. Just a bit more cautious than we used to be. We are unlikely to enter war zones as I've done a couple of times in the past.
The stresses of independent travel with own vehicle can be quite high at times. As we've become older (that's older, not old) we perhaps notice them a bit more. Just a case of recognising that and working with it. Its really a matter of choice whether or not to worry about what may happen when camped in the middle of (say) Mongolia. We chose not to worry.
There are lots of excuses available for those who decide not to travel as we are. All valid for the people who make them. At the end of the day we simply decided it would be nice to visit countries outside Aus in the truck and set about making it happen.
We (wife and I) are very fortunate that we both wanted to travel. Everything else is just a matter of planning and doing. And we are still talking to each other.
I don't know if it answers the question but I enjoyed writing it!

There's a blog (isn't there always) at
http://www.epicycles.com