Has it occurred to you that maybe it's just the kind of woman you go for? And if you are talking about your own personal sex partners has it perhaps occurred to you also that the main thing in common with the sex those women were having with you was, in fact, you.
I bet you didn't like reading that... the suggestion that maybe they like sex just fine, just not sex with you. You see, I'm taking something that could be inferred from the (very limited) evidence and stating it like I know it to be true. Which is what you did. The older women you have been with may have become bored of sex with you because it is boring, they may have stopped liking sex because, as the relationship progressed, you stopped treating them in the way that made them want you. You may have just got unlucky and ended up with women that have hormonal problems as they get older.
Now, I can't really make a general statement that I believe your lack of satisfying sex with women over 30 is your fault - I can suspect it, but I can't know it because I don't have all the facts. In the same way, you can't make a statement that if a man wants good sex "it has to be with someone under 25, 30 max" because you just have not slept with all the women in the world. You are taking your own experience and generalising too far. I know plenty of women who are not members of this site and who are in their late 20s, 30s, 40s, 50s and 60s who still have strong sexual appetites. There is a school of thought, a very strong one, that suggests that women do not hit their sexual peak until 40+.
I'm not shouting you down - I have no reason to doubt that your experience is as you say - but your experience is not universal. Isn't it possible that you are a big factor in the way the women you have sex with respond to sex as a relationship goes along its way?