Men aged as young as 18 should consider freezing their sperm to avoid their children suffering genetic disorders if they choose to have them later in life, according to an expert.
Studies suggest that sperm becomes more prone to errors with age, increasing the risk of autism, schizophrenia and other disorders.
And Dr Kevin Smith, of the School of Science, Engineering and Technology at Abertay University in Dundee, believes freezing sperm on the NHS would avoid the risk of "gradually reducing human fitness in the long term".
In a paper in the Journal of Medical Ethics, Dr Smith said modern genetic studies had confirmed that the sperm of older men contained a greater number of mutations and that a minority of those mutations present a risk to the health of future children.
He said: "If demographic trends towards later fatherhood continue, this will likely lead to more children suffering from genetic disorders.
"A trend of later fatherhood will accelerate the accumulation of paternal-origin de novo mutations (genetic causes of disease) in the gene pool, gradually reducing human fitness in the long term.
"These risks suggest that paternal age is of ethical importance."
He said options to counter the risk include health education to promote earlier fatherhood and "incentives for young sperm donors and state-supported universal sperm banking".
"The latter approach would likely be of the greatest benefit and could in principle be implemented immediately."
Dr Smith told BBC News: "I think on a society-wide basis, we do need to worry about it - it is a very real and pronounced effect.
"It's time we took seriously the issue of paternal age and its effect on the next generation of children."
He said people in their 40s might want to use frozen sperm and that it should be banked ideally around the age of 18.
It costs £150-200 per year to keep sperm privately, although an NHS equivalent should be cheaper to run.
Freezing eggs from women planning families when they are older is far less unusual.