Strength / Weight Training

Yes, the concern is there, hurtling towards 40 myself.

Supposedly good sleep, good diet, good fats, vitamins, weight training all help keep it topped up. Lowering stress and improving sleep is a problem, with young kids about.

I definitely notice a difference at 42. In addition to what @Acliff says, adjust workout plans to suit. Don't go too easy, but don't smash heavy volume 5 days a week. It's difficult to accept that you have to change your program. I really should start doing more stretching and mobility work too , as I'm starting to pick up strains and aches a little more

But there are people on the forum well into their 40s who have made great progress/ look fantastic .

That or start HRT .

I’m 55 and the only drop off of performance I have noticed was Covid related. I’m planning to try and improve fitness etc soon but pressures of work and family make everything difficult - especially work.
I played a lot of squash before Covid - 3 or 4 days every week for a minimum of 90 mins each day. I have played a guy who was 73 years old - so don’t worry too much about your 40s!

If you're concerned, you can get your bloods done for around £100 AFAIK.

Test boosters are an industry rife with snake oil. Be careful. The best things for testosterone are generally just things that are good for life. Good sleep, low stress, not too much booze, balanced diet.