Sex is getting harder, could this be due to low testosterone levels?

Maybe: Sorry for being vague, but there isn't enough information to really answer this question. Low testosterone in men causes decreased libido, but if the problem is a poor erection, there could be problems in the blood vessels. If this is coming from a woman, there are many reasons for decreased libido, including vaginal atrophy from menopause, emotional distance from the partner, or depression.

Testosterone : Depends what you mean by getting harder.. If you mean that your desire(libido) is diminished, you don't get early morning erections, you have difficulty maintaining an erection, then this could be related to low testosterone levels.At your age, that would be unusual but possible.If you do have some or all of the above symptoms, you should get your testosterone levels checked.

Yes, but at your: age it is unlikely. Get it checked!