It depends on where you live. The two most popular are Match.com and eHarmony. You will find all ages on these sites. Match lets you contact anyone you want on the site, but eHarmony makes the selections and sends them to you. A big free site is Plenty of Fish, but I've had no luck at all with it, but I've heard of many people meeting this way. I tried Eharmony for a month and they kept sending me much much older men. I've had the most luck with Match.com. I'm 55, BTW.
A lot of it is luck and perseverance. Well, you do need to write a good profile and have good photos posted.
I've met a couple of women from match.com and it's reasonably well run. You certainly give and receive more information than you do on plenty of fish.
Another site that I like is okcupid.com. It's free and, in my opinion, you can get much greater insight into the other people on the site. You have to write a profile (as with any other site) and then there are very many multiple choice tests that are written by other members on the site. In addition there are questions that you answer to determine your compatibility with other people on the site. Once again, answer as many or as few as you choose. What I like is that you can get a percent match based on the questions you answer. However, you can also get insight into prospective matches by what questions they choose to answer as well as how they answer them.
I think it has more of an edge than the mainstream sites do. Give it a try. The only thing you have to lose is the time it takes to fill out your profile.
A lot of dating websites buy databases or share other websites' members to pad their numbers. I was looking at match.com and spotted an interesting profile that read, "My sister told me to join datingdirect.com so here I am..." I think Yahoo Personals and match.com "share" as well. I don't know how the prices compare, though.
When your subscription is about to expire, you will suddenly get inundated with "winks," which I don't think is a coincidence.
There are also a lot of fake profiles that I've seen over the years. Identical photo, identical text, very attractive men. Sure, people move around but you think they might change a word or two now and again!
Seems to me years ago when eHarmony first came on the scene, I created a profile and they sent me two matches, both totally not my type, and one was about thirty years older than I was! Do they ever send women much younger matches, now with the greater social acceptance of "cougars?"
dRIVING FOR THE RIGHTS OF THE INDIVIDUAL SINCE 1969
Personally, I give up on Online Dating. I either meet total freakin' weirdos or young men interested in sex or a mommy, I'm not sure which.
I'm 41. I either get e-mails from 55 year olds who send me e-mail after e-mail asking me out when we barely exchange an e-mail, or contact from 25 year olds with no education and are total losers.
I've tried Match, eharmony, tickle.com, plenty of fish...you name it. I'm at the point where giving up is the best option. I think online dating has become what singles bars became in the 70's. Gross.