Do an old man's sperm look just a young man's sperm under the microscope?

Sperm of men. Microscopically there is no difference between sperm from a young and an old man. Clearly the dna of the sperm from an "old" has a higher chance of abnormalities associated with them.
Yes. There are mild changes in sperm quality with advanced age, but sperm are generally normal appearing in older men, indistinguishable from a younger man's sperm under the microscope.
They look the same. There may be fewer sperm, but you can't tell this by simply looking thru a microscope.