Billie Eilish and Jesse Rutherford's relationship has sparked controversy over their 10-year age gap. Therapists say it isn't a deal breaker.

• 21-year-old Billie Eilish's boyfriend Jesse Rutherford is 10 years older than her.
• Their relationship has gained some online criticism due to their age gap.

Billie Eilish's brother Finneas defended her relationship with her boyfriend after a TikTok user commented on the couple's 10-year age difference.

"Your sister's dating a 31 year old man and your music is shitty," the TikTok user wrote, People reported. In response, Finneas said, "I want my sister to be happy and safe and she is a 21 year old adult perfectly entitled to make her own life decisions."

Eilish announced her relationship with fellow musician Jesse Rutherford in October, with an Instagram post that appeared to be a joke about their 10-year age gap. Eilish was dressed as a baby and Rutherford, a 31-year-old singer for the band The Neighbourhood, was dressed as an old man. Neither Eilish nor Rutherford have directly responded to comments about their age gap.

Though a difference in age between partners sounds alarming to some, relationship experts say an age gap is not a surefire sign the relationship is doomed. Other factors — the life stage a person is in, the values and interests they share with their partner, and ways they connect — also impact a couple's relationship dynamic and long-term compatibility, Sara Tick, a therapist and founder of men's dating coaching service Modern Meet, told Insider.

Feeling connected to a partner regardless of age difference

If someone enters a relationship with intention, an age gap could be irrelevant. But before they invest, they should be sure there's more to their compatibility than chemistry, therapist Tammy Nelson told Insider.

"Think about why you are interested in someone so far out of your own age range, before you get too invested in the relationship," Nelson said. "It doesn't always work out and there are reasons why."

She said that a person's personality, interests, maturity level, and circumstances could make them a fit for someone else regardless of age. But an age gap could also reveal differences in humor, lifestyle choices, and hobbies, which could ultimately isolate partners, Nelson said.

An age gap is only an issue when it affects someone's ability to understand their much older or younger partner, therapist Rachel Wright told Insider.

"The only age gap that is 'too big' is one where you, as the person entering into the relationship, don't feel connected or can't relate to the person you're with. That's it. It's no one else's decision," Wright said.

A difference in life stages or lifestyles could lead to relationship issues

According to Tick, partners who are in different stages of life are more likely to have compatibility issues than partners with an age gap but similar life stages.

She said people in their early 20s are typically still in school, learning who they are, and not yet established in their careers. That could be an issue for a 30-something with a clear set of values, life goals, and financial independence. The same age gap could be fine for a 40-year-old and 50-year-old who are dating, Tick explained.

"It's not necessarily what the age gap is, but what phase of life you are in. Depending on what experiences you've had, that's really going to be the determining factor for a long-term relationship," Tick told Insider.

Knowing if a partner prefers staying in or partying with friends is another important consideration, Tick said. Age isn't always indicative of lifestyle, but it can be, she said.

She said that it can be a bit of a different situation for celebrities like Eilish who have many life experiences at young age and potentially mature faster. Tick also said someone in Eilish's situation, where she was a fan of her boyfriend's band as a teen before dating him as an adult, may want to consider how they're viewing their partner.

"Hopefully over time, after getting to know somebody the power dynamic changes a little bit and they become more on equal footing. But initially, I think it can definitely cloud somebody's judgment to be with someone they once had a crush on, or admired, or looked up to," Tick said.