As corny as it sounds, a woman's best age is whatever age the woman you're most in love with is. That's always subject to change. You could end up falling in love with an 18 year old girl because of her unspoiled optimism and how easily she is impressed with your life's work and the life you've built around yourself compared to the rest of the men in her life. You might like a sassy woman in her late twenties because she's confident, secure, knows what she wants, refuses to play games, and is smart and worldly enough to carry an enthralling conversation. You may fall in love with a woman in her thirties because of her nurturing tendencies, her loving disposition, and her years of "experience" making her an exceptional, soft, and affectionate lover. Hell, she might even be in her 40's, where you can both look back on a lifetime of mistakes, triumphs, and hilarious events that made the entire journey worthwhile.
There's a reason to fall in love with women of any age. They'll always give you a reason. They'll also give you a reason to drive you crazy, but hey, them's the breaks and are a part of what make participating in the love game worthwhile. It makes you feel alive. It reminds you that you have an ability to feel something in this crazy ass world.