Men can have children at any age...without viagra...that was done all the time in the 19th century when a 50 year old man could marry a 20 year old woman and it wasn't a big deal.
If I married a 20 year old there would be no problem with me having children...if a woman over 50 married a 20 year old man I really don't think she could have children...that's just reality, I'm not trying to create ill feelings here...
Men might be able "to have children at any age..", but it does not mean their children will be born healthy (physically, emotionally and/or intellectually), because their sperm is older. Yup, it is in fact the same as for woman... Men and women who chose to have children when they are older might give birth to a healthy child, or not. If they are willing to take this chance, it is their business, for sure. But they should be willing assume the consequences either way of their act of bringing into the world a child who will begin his or her adult years with elderly parents that might be in good health or losing their autonomy. Quite the burden to place on their child at an age when this one should be starting to experience all that life has to offer to them, and start to make their dreams come true.
Not all human behaviours are right, even if they continue to be encouraged and happen. In developped countries people are supposed to know better how to behave for being better educated and aware that human beings go through various developmental stages at different ages. Huge age difference in couples are their business, but are most likely due to psychological issues that they have.
Intergenerational attractions are normal and have existed since the beginning of time, but it does not mean they should be acted upon as in primitive times. A man or a woman strongly attracted to someone much older than they are and acting on it might have been victimized in their childhood by a sexual predator or abandonned by their parent of the opposite gender and they either do not know any better (have no self-esteem, no concept of right or wrong) or are looking for a replacement parental figure. A man or a woman who lusts after a young person and seduces them is either unaware of forgetting that they should be setting an example for that young person as to how a mature adult should behave when noble in heart: by resisting inappropriate attractions. In the olden days, people didn't know better. Nowadays, we do. Which is why it is wrong to exploit a person much younger than oneself in order to get what one desires too late in life (children), even if this person is wanting to be exploited. Christ taught us through our baptism that we are royalty. Royalty of the noble sort. This means self-sacrifice at times, even when we do not feel like it. It means we are called to reflect on our actions, to question why we want what we want and to ask ourselves, "What Would Jesus Do?" May God have mercy on the souls of those who chose to exploit youth or who chose to enter into relationships with adults with whom they have nothing in common, nothing to speaking, just physical attraction, as if this is love.