Mature Dating UK is for older singles, 45 years old or better. It is specifically for women and men living in the United Kingdom, and has people seeking romance and companionship as well as casual dating. They offer a free starter membership with limited access to features.
It is specifically for British singles though you can find people from other places too.
The site is designed to be senior friendly and allows the more mature single to meet others who are around their own age. It's for casual fun or more serious encounters. This particular site is now part of the Cupid family of sites.
Cupid sites claim to be quite successful in finding matches for couples. Indeed most people here seem to be looking for more serious, long term relationships. There are a few others too, covering various niches and preferences.
On the plus side they are feature rich, and offer you many ways to get to know and communicate with your potential match. There's also advice on how to handle yourself in an online dating situation and how to keep yourself safe. On the negative side it is a small community and I couldn't figure who was real and who wasn't.
Most mature dating sites have a membership that is slightly older than the younger end of their membership range. For example, someone aged 45-50 may find just as many matches on a traditional site not geared at more mature people as on a "mature" site. For those over age 55, however, there will likely be many more matches.