Consistency may change with aging and prostate either making more or less seminal fluid. Sperm are made all throughout life. It's possible that a random genetic mutation could happen sometime through life that would change the genetic makeup of the sperm as well (lead apron at the dentist) depending on hormone levels of FSH and LH the count of sperm in semen could change as well.

Older fathers have more mutations in their sperm, see: https://genomebiology.biomedcentral.com/articles/10.1186/s13059-016-1110-1

Most of these are harmless but rarely they can cause problems if an important gene is mutated.

Research has shown that the methylation pattern of sperm changes with age. Methylation is an epigenetic phenomenon which determines whether sequences of DNA are active or silent. The methylation changes are observed in sequences thought to affect neurodevelopment (increasing the risk of autism and schizophrenia) as well as increasing cancer risk. https://www.nature.com/articles/s41598-020-80857-2

In addition to the longitudinal changes already described, there’s also a selection effect. Fertile 60-year-olds are not a random sample of fertile 15-year-olds: they’re likely genetically predisposed to longevity and healthy aging. So you’d expect children of a 60-year-old to outlive those of a 15-year-old, holding all else constant.

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