In dating terms, this refers to a man old enough to be a woman's father.
Usually a man more than 15 years older than a woman is considered to be an older man.
Also closely related to the definition of a sugar daddy. A sugar daddy is an older man but an older man need not be a sugar daddy.
Actress Catherine Zeta Jones husband Michael Douglas is an older man.
Sonia told me that she would love to marry a rich older man.