Aikido is so flexible that it can be adapted to everyone - regardless of age, gender and fighting spirit. There are people who practice aikido because they enjoy its fighting spirit (although it is quite different from other martial arts because there is no sparring). But there are also people who practice Aikido just to be physically active. Tired of the daily duties, work and home, they come to practice to relax, recreate and meet friends.
Older people do not have a lot of options to be physically active. Aikido is one of them. Anyone can begin to practice Aikido regardless of age (except for very small children - ages vary from dojo to dojo) as long as he/she is ready to practice and learn. But that doesn't mean that there is no place in Aikido for young and strong people who want something that is more in the spirit of a martial art.
Aikido is something that you can practice your entire life. Aikido is free from the pressure of competition and achieving results. This is not about a comparison with other people and their capabilities, but about improving yourself. So there are many people who practice Aikido for more than forty or fifty years, but also those who are starting to practice when they retire.