Ok, this turned out to be long. I've a very low libido these days, but there are oh so many factors as to why. Wouldn't bother me if I never had sex. No desire to masturbate either.
I sometimes find it all a little crass and animal-like. It's hard to describe it. I hate feeling like an object and am almost ashamed of myself for deriving sexual pleasure. I suspect there is also some residual Catholic guilt involved.
I think sexual assault in the past has affected me in a way I've never bothered to explore in therapy which might be why I find it such a base thing.
Prior to this, an extreme fear of getting pregnant was a huge factor. I cannot tell you how big of an issue this was. Nothing like the thoughts of an unwanted pregnancy to cool your ardour.
When I am fit, exercising and slim and strong, I've more desire with a new partner. Partly due to body confidence and partly because my body is more capable of the gymnastics required for me to personally orgasm. I also suspect exercise and fresh air might increase whatever hormones make you desire sex. When I'm unfit, blobby and look like shit, nope, no desire. Have any of you ever cramped while in the throes of passion lol? I seize up like an old car which hadn't been brought out for a while. On more than one occasion in the past my moaning and involuntary spasms due to cramped feet and legs has been mistaken for a powerful orgasm. 
Sexual coercion and coming across like a dog with two bugeens was a guaranteed buzz killer in the past. Fuck off. Get the hell away from me with your grubby paws.
I am far more likely to feel desire when I find it believable that someone desires me. By that I mean that when I'm single and dating, I'll have shaved, moisturised, painted toe-nails, put on tan, dressed up, had hair done and had make-up done. I feel far more attractive then so am more likely to feel desire as I can believe that a man does actually desire me. By contrast, in a comfortable long-term relationship where I come home, the bra comes off, the g-string comes off, the big knickers comes out along with the socks and baggy loungewear. For some reason I don't in such instances find it believable that my partner could actually find me attractive and I feel like it's a case of 'any hole will do'. That doesn't turn me on.
Weather! In Winter and the cold, I wear the warmest, most snug and comfortable clothing possible to bed (including socks). A veritable yeti. In Summer however, where I wear less clothing in bed and I suppose the 'skin to skin' contact contributes to arousal perhaps. I suppose this is why Italians and those with warmer climates are more renowned for their libido.
Like a pp, different timings for libido. Wake me up from my precious sleep fondling me with something poking me in the back and you'll soon fucking regret it. I cannot stand it whereas other women find this a turn-on. We had a term for my midnight or dawn reactions to an uninvited come-on - 'the asses kick'. He would quite literally get a backwards donkey kick to get him away from me. Being a resilient sort it never thwarted him long-term, but that was just that relationship really lol. I'm not generally in the habit of physically assaulting boyfriends (he exaggerated that it was a kick - it was moreso me moving him away from me with my legs). On the other hand, when I do have desire is after a few drinks. This is fine once the man has no issues with erections after alcohol. Men often seem too tired by the time I'm up for it and not only that but roll over and snore like a big right after the act. Ew. Likewise, when I'm out of the shower, make-up done and am getting ready for work or to go out, do not pick such a time to want to mess up my hair, makeup and sweat all over me. Pick a better time dude.
I can't say that I'm on medication that is known to affect libidio but I have been in the past which might have been a factor at times.
There has been a noticeable diminishing of my libido over the years but there are so many factors involved, none of which I would attribute to biological aging factors exactly. Yes as I've mentioned, there are physical elements, but that's not age-related. When I was fat and younger I had as much desire then as as do now when I feel like shit.
I'm also far less likely to simply not entertain the idea the older that I've got. When younger, I would have been more open to sexual advances when not aroused as I knew that I would become aroused in the process.
I have no peer reviewed evidence for any of the above. They are quite simply my own very personal experiences and surmisations.
In summary, start working out, do some yoga, get loads of fresh air, move to a sunny climate, wear little or nothing in bed, get that rusty razor out and shave groom, doll yourself up so that you like what you see in the mirror, have a few glasses of wine and off you go!
Or you could resign yourself to a sexless existence like moi