I am now 40. I did some weight lifting about 15 years ago and it was fun. I am now back into it, focusing on strength and muscle gain. I obviously don't expect to become a Mr Olympia, but I find it much more fun to do the exercises with the goal in mind of becoming stronger and have bigger shoulders.
I don't know if it might be a subjective impression, but I don't remember to need so much time for recovery after a hard training (on the 3rd day after a shoulders and deltoids training I still feel it).
Is there any kind of relation between the time needed for recovering and the age? That is important: Your muscles aren't supposed to grow if you don't allow them to rest long enough, namely at least a couple of days between trainings when you are in your 20s. If the times needed are longer (then, how much longer?) for a man in his 40s, then the usual routines for strength and muscle gain will not work, and you need to adapt them somehow.
Additionally, any other clue about how to adapt the routines to a somewhat aged body (well, apart from being more carefully and progressive with the increasing weights of course) are welcome. Might it work if I simply use the normal workouts and insert additional rest days (or days of soft aerobic work) in between?
From the answer below by Berin Loritsch, and also from this other answer I am gradually becoming aware of the following thing: at least at the age of 40 it will be much more useful to stop worrying about it and simply go to the gym and work your butt off like hell!
But anyway, the question is interesting, and I am willing to accept the most useful answer.