Only the people involved can decide if age is a problem in their relationship. In all honesty most of the conversations we have with friends, co-workers, family, spouse, or significant others have nothing or very little to do with age perspective topics.
Generally speaking the majority of conversations are about what was on TV last night, weather, traffic, something that happened at work, vacation plans, what you did over the weekend, relationship drama, worries, good news/bad news, or how someone feels.
It's very rare people sit down and say, "Remember when... or Where were you when...?" This probably explains why younger and older co-workers are able to coexist and cultivate friendships. (When we like someone we tend to look more at what we have in common than what are differences are). An older and younger person might enjoy the same types of books, movies, restaurants/foods, hobbies, traveling, going to the beach, skiing and other outdoor activities. Also older people strive to look and feel younger while younger people often desire to prove they are smarter and more mature beyond their years. Personally speaking when I turned 18 I thought I was "grown" and considered myself to be "equal" to the adults.According to the legal system I was an adult and that was enough for me....etc I imagine most 18, 19, 20, and 21 year olds feel the same way.
I suspect the major disadvantage to older and younger relationships is the health related ticking time bomb. For much of soicety the older we become the less active we are, the more medications we take, the higher risk of strokes and heart disease...etc A 40 year old with a 20 year old may not have any issues but if they stay together when they become 40 and 60 or 50 and 70 the odds are the younger person will either become a care giver or have to place older person in a nursing home. (Unless he/she is rich like Hugh Hefner, age 84 and datiing 24 year olds). :-)