I think the rule of half plus 7 works more often than not because it tends to keep people grouped with other people who are going through a similar stage of psychosocial development, or just one stage away. That means that goals, interests and challenges are similar enough that you likely to move in the same general direction and also make you compassionate and understanding with the issues each other are facing. While the ages of these stages can vary, chances are good that you’re both smack in the middle of the same stage, so I wouldn’t be unduly concerned with the age difference.
Where things can get sticky is when one of you is still figuring out, for example, Identity, and the other is ready to get on with Generativity. One wants to party with friends till 4am every night, the other’s ready to settle down and start having kids and staying in. That can lead to some pretty obvious conflicts that may not be reconcilable.
At the other end of life, there’s also a concern with very large age spans and one partner simply being unable to keep up, physically speaking, and facing increasing dependence and health challenges. It can be rough on the younger partner dealing with all that - often at the same time as they’re dealing with their own parents’ decline. It can also be rough on the older partner, feeling like they’re asking too much of the younger, or that their young partner would be better off with someone more their age. But a 5 year difference - honestly, that’s not likely to be a big factor, even if this is The One. I’m facing that with a 22 year age difference (I’m the younger, and he has even more health issues than most 60 year olds). I wouldn’t give him up to make my life easier, though.
For reference, Erikson’s stages of Psychosocial Development:
Approximate Age: 13–19 years
Psycho Social Crisis: Identity vs. Role Confusion
Significant Relationship: Peers, Role Model
Existential Question: Who Am I? What Can I Be?
Examples: Social Relationships
Approximate Age : 20–39 years
Psycho Social Crisis: Intimacy vs. Isolation
Significant Relationship: Friends, Partners
Existential Question: Can I Love?
Examples: Romantic Relationships
Approximate Age : 40–64 years
Psycho Social Crisis: Generativity vs. Stagnation
Significant Relationship: Household, Workmates
Existential Question: Can I Make My Life Count?
Examples: Work, Parenthood
Approximate Age : 65-death
Psycho Social Crisis: Ego Integrity vs. Despair
Significant Relationship: Mankind, My Kind
Existential Question: Is It Okay To Have Been Me?
Examples: Reflection on Life