I tend to look at relationship age in maturity in brackets. People usually fit into them according to Erikson's Stages of Development, and relationships really only work (between the two of them and as far as society sees them) when two people are in the same arena of development when the relationship begins. For instance, people in the Adolescent - Identity Crisis stage do well together, or people in the Inimacy Crisis stage do well together, but not when one is on the other stage.
Adolescent
Identity vs Role Confusion
Tries integrating many roles (child, sibling, student, athlete,
worker) into a self-image under role model and peer pressure
Young Adult
Intimacy vs Isolation
Learns to make personal commitment to another as
spouse, parent or partner
Middle-Age Adult
Generativity vs Stagnation
Seeks satisfaction through productivity in career, family, and
civic interests
Older Adult
Integrity vs Despair
Reviews life accomplishments, deals with loss
and preparation for death
Age is of general consequence to these stages, but if someone is in one of these stages too soon or too late, it may seem offputting to society. However once people get to know a couple, they just seem to 'work', and that's why.