I don't think that the capacity for love diminishes at all as we get older. I think that the practicalities of finding love make it more difficult as we age. By that I mean that people collect baggage. By the time you get to my age many people are or have been married, have children, jobs, committments, have been hurt by bad break ups and have closed their hearts to love.
Also people become more conservative as they grow older. Those who were rebellious in their teens and 20's who were punks, goths, potheads, ravers now wear suits and have families. My sister told me recently that I am never going to find someone like me because they will all have grown out of it by now. I don't think that's true but I do think that he/she will be more difficult to find now.