Sometimes you have to take a chance in life.
From what you are telling us, you are able to talk to this lady on just about any subject, including sex so if you can talk about sex to her, why is it difficult to admit to her that you find her sexually attractive? She is hardly going to bite your head off the telling her this, in fact, she will probably feel flattered to know that a younger man finds her attractive and desirable. No woman would feel offended by this.
Once she knows that you find her attractive, you are putting the ball in her court, to use a tennis metaphor, and it is up to her how she plays that more. She may turn around and say that she has never thought of you in a sexual way, and so you can take it that she is not interested. She may go quiet, and she may even scold you a little, but she is unlikely to break off all contact with you for admitting that you find her desirable.
What you have to consider is that, even if she isn't enjoying a full and satisfying sex life, it doesn't necessarily mean that she is ready to jump into bed with the first guy that shows any interest in her. You point out that the two of you are close and friendly terms, and she may value friendship is far more important than sex stop on the other hand, she may consider that, given that she already likes you as a person, perhaps she would like that friendship to extend into a more physical relationship. The thing is, you will never know until you talk to her on the subject.
You don't have to be crass about this and come out with something like "I really fancy you would want to take you to bed." All you need to do is to tell her that you think she's beautiful, and that you cannot understand why she is on her own. This opens the conversation, and then it is up to the two of you where that conversation takes you.
Suppose for a moment that you make a statement like that which I have suggested, and it results in the two of you finishing up having sex. Where does it lead to in the long run? It's okay starting a sexual relationship with somebody, but you have to consider that people are not robots, they are full of emotions, longings, desires, hopes, and fears. You will both start something, and you will both have to consider what the consequences of an intimate relationship can be. Okay, she's older than you, but that doesn't need to make any difference. If you fall in love with each other, you would probably meet with resistance from your family and hers shouldn't make any difference if the two of you become romantically attached. She's not that much older than you that it would make any difference, and as they say, love is blind many couples have a very loving and happy marriage, even when the age differences are quite significant. Convention says that an older man can marry a younger woman, yet society looks askance at an older woman marrying a younger guy. There is no real reason for this, except that it challenges the age-old traditions of the dominant male and the submissive female. A mature woman doesn't fit into their neat categories, and so it is regarded unfairly.
You notice that I'm talking about love and relationships here, but this is what you have to consider when you start engaging in a sexual liaison with anyone. The only time you can isolate sex from emotions is when you visit a prostitute. Prostitutes engage in sex as a purely financial transaction, and whether they enjoy it or not, there is no emotional component attached to it. The moment you start having sex with somebody that you like, the future is unpredictable. You have already admitted that you like this lady a lot, and a sexual relationship may make your feelings far stronger for her. What would you do if your feelings are not reciprocated? On the other hand, you may regard having sex with her is just some pleasurable entertainment, yet she may see it as part of a far deeper relationship, and what would you do she falls in love with you? If one or the other has feelings that the other doesn't share, then any kind of relationship is doomed to failure, and both parties getting hurt in one way or another. So while you might think it's fun to have sex with this lady, bear in mind that there may be a lot more to it than just a bit of sexy entertainment
I'm not saying that you shouldn't do this, what I'm saying is that you must think first, and act after you have weighed up as many possibilities as you can. But before anything else, you have to break the ice. You can tell her that you think she's beautiful, and that you can't understand why she is still unattached to anybody, without actually pushing yourself forward to become her partner. If you tell her these things, as I said, you leave it up to her how she wants to react. Just think carefully first!!!
I hope this has made things easier for you. It does put the onus on you to make the first move, but you don't have to do anything stupid that will result in a potentially angry confrontation. Just engage your brain before you engage your mouth, and I think you will be okay stop