It may not suit their too-young ideas about how they should look or feel, but that's because their ideas haven't moved as fast as their bodies. Many of us in our 70s, 80s, and 90s find, whether we have a partner or not, that our sexual imagination is as vivid as ever. We may wonder what the psyche is doing by sending us sexual fantasies when we're alone or impotent, for example, but these thoughts and images keep a person's spirit alive. He is not ashamed or self-critical, thinking he shouldn't be having such ideas when his body no longer functions the same way. That is the biological view, that if there is no sperm count, or you're not ovulating, erotic life serves no purpose.
Old people walk down the street and have sexual fantasies all the time.
You see people on the street and you think they're getting it on with one another. It keeps us erotically connected to the world. If you aren't enjoying your imagination for its own sake, that can be a big problem. But it's not nearly as frustrating as you think it will be when you're 50. At 50, you're carrying the ideas of your sexual youth quite closely.