Around 40 your body starts to deal with exercise differently due to changing hormones. This goes for both men and women.
That means that if you continue to follow the same regime you had when under approx. 40 yrs of age you are likely to gain weight.
This is because the body deals with that type of exercise (think long runs, cycling etc.) in different ways and because it doesn't know how long it will have to keep doing that it starts to create fat as a reserve.

So that means that after around 40 you have to shift your way of exercising. Apparently the HIIT type of training is much better. Anything that involves several large muscle groups can help balance hormones.
I know that power training helps produce oestrogen, which can also be made by muscles that way. Maybe the same goes for testosterone?

I got this from Gregg Braden, this vid, approx. 9 mins in:
https://www.youtube.com/watch?v=x02fJKO6Tsk

I noticed a big difference within a couple of weeks of switching to HIIT Training in my early 50s. The exercises that had kept me trim from my 20's to my 40's were just not doing as much. Two weeks into HIIT training I began to lose weight again.

HIIT is good for working out, but I would not totally rely on that with being older. Mix things up so you are not doing the same thing everyday. Swimming is another all body workout if you do it right.

Something to think about is your joints (elbows and knees, not the rolled green stuff...lol). One idea is SLOWER reps, you can get the same burn with less weight. Say you can do 20lb bicep curls that take about 2 seconds per curl up and down, do them with 10lb and make it 6 seconds up and down per rep.

Never heard of it but I'm going to mix this with other workouts.

Not a surprise, this is from relatively new research :)
It does make a lot of sense. You could watch the vid to have it properly explained if you wish :)
Link is in my OP (copy paste it).

This is how strength training stimulates the production of testosterone, as well as the production of growth hormone, and so does dry hunger, contrast breathing and light sleep. Let's say it gives me a lot, young people are not able to do as much as I can, I didn't want to brag, I wanted to tell about the result :)