If age is just a number, then how old would 20 years "her senior" or "junior" be? Is gender an issue?

Answer 1

20 years her senior means he's 20 years older than her...and 20 years her junior means he's 20 years younger than her...

Answer 2

Age isn't always just a number. 20 years difference doesn't matter much if the numbers are 40s - 60s or 50s - 70s, etc... But if it's under 40 then it tends to present a problem. Gender is always an issue I'd say, but men like to go for younger women when they hit a certain age and women are more prone to go for an older man rather than a younger one, despite any attraction.