I'm not sure about the 'taking longer to cum' situation in regards to low T, but that could be effecting your desire the following days.  I'd check with your primary care physician to get his/her opinion.

Sounds like a perfectly normal age progression to me.

I think there are obvious changes to us physically as we age which could lead to the problems you're having.  See your physician as the other poster suggested or search online about ejaculation problems in your 50's.

I think there's also a mental part of your question which can also contribute to your lack of desire.  That's a question only you can ask yourself to answer.

Not when your with me. ! maybe the chemistry is off. ...

we usually have mutual orgasm though, Maybe you need the chemistry to be there more. I know I would like marry you in real life your so godam sexy.
god my heart broke when I found out you were taken,  

 
                  I bet that doesnt happen with me, sexy boy.......but i get really horny when i look at you so maybe that turns you on more.

your fuckin hot so I cant imagaine other girls not having legitimate attraction as well when they fuck you.

you kinda look like a pro baseball player, then the face too is just, mmm striking, yummy yummy.  

 
xoxoxox