I'm 36 years old. I do (mostly) full body 5/3/1, and I thought at first that it wasn't hard enough. I was hesitant at first, because it seemed too easy. But everybody I read who tried it said it works, and I didn't have anything to lose, so I tried it too. In February, I benched 225 for 9 reps, which was an all out maximal effort after a few months out of the gym. Last week I did 275 x 10 during my '3' week, and left at least one rep in the tank. This week I did 290 x 6 after working up to 325 and 335 for singles (*I know, it's not in the program). I'd never, ever, ever benched 315 before I started 5/3/1; now it's an easy triple.
The reason it works, I think, is that it eliminates training inertia. How many people do you know who go into the gym and load the same weights on the bar week after week after week? I was one of them. You can get stronger that way, sure, but it's a slower adaptation because you rarely provide the body with new and different stimuli. 5/3/1 shows your body different stimulus every week- you get aerobic stimulus, increased load, a reason not to sit around between sets. Then you do work to make you better at the big 4 lifts. It's simple and easy. It keeps you from being "too smart" for your own good, too. You go in, do the prescribed sets, hit the last set hard, do some accessory stuff, and go home. No changing lifts every three weeks because you don't feel like it's working (as if three weeks is long enough to tell).
I will note that if I followed 5/3/1 to the letter, I still wouldn't have benched 315, because it doesn't show up in the percentages until a couple of months from now