gRegor, I don’t think that it’s necessarily that it’s harder to make friends as an adult, so much as the world you live in during your youth makes things comparatively much easier.
When you’re young, you’re generally in school(s) where you’re around people exactly your age, generally close to your socio-economic status, and with many of the same feelings, thoughts, and aspirations. You’re literally surrounded by hundreds (or sometimes thousands) who are so very similar to you. Once you’re out of college, it’s far harder to find this type of environment and this is what makes it seem so much harder to find good friends. In adulthood almost everyone you’re surrounded by are dramatically different from you and that makes it harder to find things you have in common. In the end it’s really the statistical mechanics that are working against you.
To work against this one needs to be more flexible and broad in what one is looking for in companionship, but generally the older one gets the less flexible one becomes.