Lifting at age 40 and beyond

I'm almost 42. I started exercising hard and lifting (combo of heavy and endurance workouts) when I was in college and have done so off and on since then. I did P90x over the winter, and I was never tired from that. I had more energy during the day. I started stronglifts 5x5 about two months ago, and this is the heaviest lifting I've ever done. I definitely get way more fatigued from it than any other activity I've ever done. I'm sooo happy to be able to sleep an extra hour on my off days.

If I get some random soreness, I've found that rolling my muscles out with a foam roller or tennis ball helps almost instantly.

I am 49. I've exercised my whole life, with breaks from time to time. In my 20's, I would tear it up with abandon in the gym, on the track and I never stretched. By age 35, I had to have back surgery. The surgeon who literally saved my life told me at the time, "I will get you through this but you have to stretch every day of your life unless you want to end up a 40 yr old cripple". I am much gentler on my body, I stretch before and after I exercise whether I am lifting or running and I give my body more recovery time. I go for form and function over weight and distance. I have fortunately had no recurrence of injury since that time, 15 years. The body is adaptable but needs to be treated kindly and gently for best response.